Yorkshire v Lancashire, Headingley, Leeds, 6.30
Headingley is sold-out for the blockbuster Roses match between third-placed Yorkshire and Lancashire, last season's beaten finalists, who lie seventh. In a round replete with England players, Yorkshire possess a squad teeming with international quality after Joe Root and Adam Lyth have been released to join the likes of Jonny Bairstow, Tim Bresnan, Adil Rashid and Glenn Maxwell. Andrew Hodd and Will Rhodes miss out on selection.
Lancashire do welcome back Jos Buttler but are without James Anderson who was not released by the ECB. Alviro Petersen is also not in the squad having attended a Cricket South Africa player-of-the-year event on Wednesday. Tom Smith is left out.
Northamptonshire v Worcestershire, The County Ground, Northampton, 6.30
Worcestershire add international pairing Moeen Ali and Saeed Ajmal to their squad, with Sachithra Senanayake ending his brief stint at the county. Northamptonshire meanwhile welcome Olly Stone back into the squad after an injury for what will be Shahid Afridi's last match with the county before he departs for the Caribbean Premier League. Both Northamptonshire and Worcestershire have played just two games, with Northamptonshire having lost them both and Worcestershire winning one and losing one.
Nottinghamshire v Leicestershire, Trent Bridge, Nottingham, 6.30
Leicestershire will be pleased with their start that sees them sitting fourth in the table after four matches but they face a tough challenge away to Nottinghamshire who have added Stuart Broad and Ben Hilfenhaus to an experienced and high-quality squad already boasting Alex Hales, James Taylor and Darren Sammy.
Leicestershire welcome overseas signing Umar Akmal for his first match of the season. Akmal comes into the T20 Blast on the back of some excellent form for the Lahore Lions in the Super 8 T20 in Pakistan. Andrea Agathangelou and Matthew Boyce also come into a 14-man squad that is without Kevin and Niall O'Brien who are on international duty as well as Grant Elliott whose spell at the county is over.
Derbyshire v Durham, The 3aaa County Ground, Derby, 6.30
Derbyshire will be looking to build on their impressive victory over Lancashire last week when they host Durham. Durham have named Ben Stokes in their 14-man squad but are without Calum MacLeod who is on national duty and have left out Graham Onions and Barry McCarthy from last week's squad.
Derbyshire meanwhile have named Nathan Rimmington in their squad for the first time after his introduction was postponed by injury. Rimmington replaces Hashim Amla in the squad. He has also ended his stint at the county.
Kent v Gloucestershire, The Kent County Ground, Beckenham, 5.30
High-flying Kent, who have won four from five, host Gloucestershire, who haven't played since Michael Klinger's hundred carried them to victory over Essex in Game Week 2. Kent have added Matt Hunn and Ryan Davies to their squad that beat Somerset and have left out David Griffiths. James Tredwell is still out with injury. Gloucestershire meanwhile haven't included Robbie Montgomery in their squad.
Somerset v Hampshire, The County Ground, Taunton, 5.30
Hampshire moved to second in the South Group with a strong win against Middlesex on Thursday and will take a big stride towards the quarter-finals if they can win a tough fixture away to Somerset. Somerset have added Tom Groenewald to their squad from their thrilling defeat against Kent. All eyes will surely be on Chris Gayle who has plundered 243 runs from his two innings in the T20 Blast this season.
Glamorgan v Middlesex, The SWALEC Stadium, Cardiff, 6.30
Both these counties have shown glimpses of promise this season but both have been beaten twice and a third defeat could quickly turn a promising start sour. Glamorgan, who have won the last four encounters between the two sides have not yet named their squad. Middlesex, meanwhile, have added Kyle Abbott to the squad that was beaten by Hampshire on Thursday.
Surrey v Essex, The Kia Oval, London, 6.30
Only five matches into the season it is already approaching must-win time for Essex who are at the bottom of the South Groups having lost four matches. Essex have, in fact, lost eight of their last nine matches having won all of the eight preceding that. They face a Surrey side also in need of a win having lost two of their three matches.
England Test captain Alastair Cook will make a rare appearance for Essex having been released by England and is joined in the squad by Aron Niijar and Jamie Porter with Nick Browne, Mark Pettini and Callum Taylor making way. Surrey have added Aneesh Kapil to last week's squad and may add Gary Wilson depending on the outcome of Ireland's Intercontinental Cup match against the UAE which is taking place in Dublin.