Some scheduled Covid vaccination appointments are being rearranged as Wales prepares for extreme heat to hit.
It comes as one NHS boss said amber heat warnings could lead to the NHS being "overwhelmed" if unprepared.
Some jabs due in the next few days are being rescheduled by Betsi Cadwaladr health board.
Transport for Rail (TfW) has warned against all but essential travel with record-breaking temperatures expected.
"There is only so much we can do," said health board executive medical director Nick Lyons.
"There is a very real danger that we could become overwhelmed if we're not ready and people themselves aren't ready," he told BBC Radio Wales Breakfast.
The health board said anyone with a "booked appointment is advised to attend as planned unless contacted".
"Any decision to close a vaccination centre or rearrange appointments is based on health and wellbeing of the public and our staff," it said.
On Saturday, temperatures are expected to reach the mid-20s, but on Sunday this could go as as high as 33C (91F) in south-east Wales.
The highest ever recorded temperature in Wales was 35.2C (95.4F) in Hawarden, Flintshire, in August 1990.
But this is expected to be broken on Monday as temperatures look set to soar to 38C (100F) in some areas in the east such as Monmouthshire.
On Friday, the UK government declared a national emergency as the Met Office issued an unprecedented red alert for extreme heat, meaning there is a risk to life as temperatures could hit 40C (104F) in large parts of England.
TfW advised customers not to travel in the areas covered by the red weather warning, and to only make essential journeys in areas covered by the amber warning.
"Rail services are expected to be significantly disrupted, particularly in the borders region, where services on routes within the areas covered by the red weather warning will be cancelled," it said.
"Services in other parts of the network, including throughout Wales, are also expected to be affected."
TfW routes affected by cancellations include Shrewsbury-Birmingham, Chester-Liverpool, Chester-Manchester, Chester-Crewe, Crewe-Manchester and the Conwy Valley Line.
Jack Pitt, 23, from Tywyn in Gwynedd, is currently in Ibiza, Spain, for a week-long holiday with three friends.
They were due to travel back to Wales by train on Monday after arriving at Manchester Airport, but now faces having to find alternative transport.
"It's frustrating because we already had problems getting to Manchester with cancellations on the train from Tywyn to Machynlleth," he said.
"And a further delay leaving Shrewsbury station. This meant my friend and I had to pay an extra taxi fare of £30.
"Now I've just heard that the return train journey we've already bought tickets for has been cancelled, with no trains travelling back to Tywyn at all on Monday. And there's no replacement bus service."
Hundreds of people gathered in Llanberis on Saturday as the village hosted Ras Yr Wyddfa (Snowdon Race) up the mountain for the first time in three years.
Race organisers warned runners to bring plenty of water as they lined up in the mid-afternoon sun, with water stops placed along the route.
Ruth Evans from Caernarfon said: "I've done the race once before and it was raining and windy that time, so this time it will be nice to see the views - it'll be lovely at the top."
Organisers of Rabbit Run, held in Merthyr Mawr, Bridgend county, earlier on Saturday, had advised competitors to take it easy during the race.
There were no major casualties among its more than 500 competitors, organisers said.
Concerns have been raised for wildlife after river water levels dropped due to a lack of rain, with fishing bans introduced in parts of Wales.
On Saturday photos showed a normally submerged bridge at Llwyn-onn Reservoir, Merthyr Tydfil, exposed as temperatures rose.
Welsh Water called on people to "keep hydrated but please use only the water that you need and don't waste it".
"Our teams are out and about working around the clock to fix leaks on the network and over 40 tankers are working hard to take water where it's needed the most," it said.
Meanwhile, many councils told BBC Wales they had given schools yet to break up for the summer formal guidance from Public Health Wales and the Welsh government.
Brynteg School in Bridgend said it was including breaks during lessons, allowing pupils to wear PE kit and moving lessons to cooler parts of the school.
Chris Cousens, RNLI water safety lead for Wales, West and Isle of Man, urged people planning a trip to the beach to chose those with a lifeguard and, if they planned to go into the water, to use dedicated zones.
And the Forever Hound Trust, which rescues and rehomes greyhounds and lurchers has advised people against taking their dogs to see them at a festival in Chepstow, Monmouthshire, on Saturday due to the weather.