Men's and women's 100-ball matches are to be trialled in September before a new competition set to start in 2020.
The new format, proposed by the England and Wales Cricket Board (ECB), is a variation on Twenty20, with 100 balls per side instead of 20 overs.
Nottinghamshire's ground Trent Bridge will host pilot games from 16-18 September, with all current male players invited.
Loughborough will host the women's games on 14, 15 and 27 September.
Worcestershire batsman and Professional Cricketers' Association (PCA) chairman Daryl Mitchell said: "The current plan is to have three or four XIs to maximise the investigations on the three days while making sure the workload for players is manageable.
"We are hopeful the majority of counties will have a representative, although with some counties having first XI and second XI fixtures it will not be possible to have a player from each of the 18 counties.
"All current male players have been contacted to notify them of the details for the pilot matches, with player availability co-ordinated via the counties and the ECB."
The trials will enable players to provide feedback to the ECB on the 100-ball "countdown" format - after some had expressed concerns about the plans.
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The concept, devised to attract new audiences to the sport, would feature 15 traditional six-ball overs and a final 10-ball over - 20 deliveries fewer than a team's innings in a T20 game.
It would involve eight teams playing 36 games over a 38-day summer window, with an Indian Premier League-style play-off system and players' draft.
Lord's and The Oval would host newly created teams in the competition, with the Ageas Bowl, Edgbaston, Headingley, Old Trafford, Sophia Gardens and Trent Bridge the other grounds chosen in a "city-led" project.
The new five-week competition would take place alongside the existing T20 Blast, which the ECB says will be unaffected by the proposed format changes.