Match ends, Cardiff City 4, Derby County 0.
Cardiff City made it 10 games unbeaten under Mick McCarthy and moved back into the Championship's top six by thumping Derby County in the Welsh capital.
Leandro Bacuna's calm finish put Cardiff ahead, before Kieffer Moore doubled their lead with a header from a corner and Bacuna fired home a third.
Will Vaulks then smashed home from 25 yards in injury time to add gloss.
Cardiff's night was only tainted as Joe Bennett was taken off on a stretcher with a serious-looking knee injury.
The victory for McCarthy's side, who have now picked up 24 points from 30 since he took charge, moves the Bluebirds back into the play-off berths, while Derby remain 18th after a meek display from a much-changed side.
Derby boss Wayne Rooney altered over half his starting XI from the Rams' draw with Nottingham Forest on Friday and only one of those changes was enforced, with goalkeeper David Marshall missing out on a South Wales reunion due to injury, with Kelle Roos replacing him.
Rooney handed a first start to Everton loanee Beni Baningime, who started along with fellow January signings George Edmundson and Teden Mengi, while Lee Gregory and Louie Sibley also earned recalls as Nathan Byrne, Max Bird, Jason Knight, Martyn Waghorn and Colin Kazim-Richards dropped out.
Bluebirds boss McCarthy made two changes to the team that drew at Middlesbrough, with Harry Wilson and Josh Murphy dropping out, and Sheyi Ojo and Bacuna coming into a side full of confidence.
Derby's lack of familiarity was evident in the early exchanges as Cardiff continued the fine form they have been routinely showing since the appointment of McCarthy.
Striker Moore missed two huge chances in the opening 20 minutes, first firing over with a close-range volley after Derby dismally defended a long throw from Vaulks, before he headed across goal and just wide after Ojo had gone clear and seen an appeal for a penalty for handball waved away.
A goal for the hosts seemed inevitable and duly arrived midway through the first half when Bennett's perfect pass split the Derby defence and Bacuna ran clear before slipping the ball under Roos.
The Bluebirds were rampant and Moore fired just wide from 25 yards before a lengthy stoppage saw Cardiff defender Bennett taken off after collapsing, unchallenged, with what appeared to be a knee injury.
Aside from a long-range effort from Graeme Shinnie that Dillon Phillips palmed wide, the Rams barely threatened Cardiff's goal in the first period while the Bluebirds could and should have gone further ahead.
Ojo shot straight at Roos after a sloppy back pass and the goalkeeper then produced a fantastic double save to deny first Moore and then Bacuna's follow-up effort.
However, any strong words from Rooney at half-time became academic when the visitors fell 2-0 behind just three minutes after the break when Moore was given too much time and space to pick his spot with a free header from Vaulks' corner.
Moore has now scored 16 goals for Cardiff this term with eight of those goals coming in McCarthy's 10 games.
Bacuna then tested Roos from a 20-yard free-kick before beating him from an almost identical spot, when Ojo played the ball into his path, this time curling the ball home.
Derby offered little to suggest they could even score a consolation goal, though Phillips was forced to save Waghorn's free-kick as a slew of substitutions stemmed the flow of Cardiff chances.
However, Vaulks fired home a fourth when Derby gave him time and space in the last minute of injury time to allow him to lash home from long range with a dipping shot that just crept under the crossbar.
Cardiff City boss Mick McCarthy told BBC Sport Wales:
"I don't know if that's the best we've played, it is another good performance, we were very organised and disciplined and didn't allow Derby to play through us.
"We worked exceptionally hard and that gives you the foundation to create chances and I was disappointed we weren't more than 1-0 up at half-time.
"It was great to get the second goal when we did and we thoroughly deserved the victory.
"We'll be back in tomorrow and recovering. It's easier to recover, easier to want to play. everybody wants to play, things are good. Long may that continue, we've won nowt yet, that's for sure."
Derby boss Wayne Rooney said:
"It is a bitterly disappointing result. We had a bit of control in possession, but it didn't translate into the final third.
"It is not nice to concede four goals, it is something we will look at.
"We will now prepare for Saturday which is a big game for us.
"We made the changes as we had been monitoring the players and felt it was the right time to give them a rest.
"It was a gamble to do that, I know that and unfortunately it didn't pay off."
- 21PackSubstituted forRallsat 87'minutes
- 3BennettSubstituted forBaganat 39'minutes
- 27OjoSubstituted forWilliamsat 87'minutes
- 10MooreSubstituted forColwillat 77'minutes
- 29M Harris
- 2WisdomSubstituted forMacDonaldat 70'minutes
- 7JozwiakSubstituted forWatsonat 69'minutes
- 4ShinnieSubstituted forBirdat 69'minutes
- 17SibleySubstituted forWaghornat 69'minutes
- Dean Whitestone
- Shots on Target
Second Half ends, Cardiff City 4, Derby County 0.
Goal! Cardiff City 4, Derby County 0. Will Vaulks (Cardiff City) right footed shot from outside the box to the top left corner. Assisted by Perry Ng.
Kornell MacDonald (Derby County) wins a free kick in the defensive half.
Foul by Joel Bagan (Cardiff City).
Substitution, Cardiff City. Jonathan Williams replaces Sheyi Ojo.
Substitution, Cardiff City. Joe Ralls replaces Marlon Pack.
Corner, Derby County. Conceded by Aden Flint.
Attempt missed. Martyn Waghorn (Derby County) left footed shot from outside the box is too high. Assisted by Beni Baningime.
Martyn Waghorn (Derby County) wins a free kick in the attacking half.
Foul by Sean Morrison (Cardiff City).
Corner, Derby County. Conceded by Joel Bagan.
Attempt blocked. Martyn Waghorn (Derby County) left footed shot from outside the box is blocked.
Beni Baningime (Derby County) wins a free kick in the attacking half.
Foul by Aden Flint (Cardiff City).
Martyn Waghorn (Derby County) wins a free kick in the defensive half.
Foul by Marlon Pack (Cardiff City).
Substitution, Cardiff City. Rubin Colwill replaces Kieffer Moore.
Corner, Derby County. Conceded by Dillon Phillips.
Beni Baningime (Derby County) wins a free kick on the right wing.