Zak Crawley made his third successive sub-30 County Championship score for Kent after Northamptonshire ran up a record 684 for two declared at Canterbury.
After opening day hundreds for Ricardo Vasconcelos, Luke Procter and Calvin Harrison, Nathan McSweeney got to three figures on Saturday as Northants' top-four all made centuries for the first time ever.
Procter made his a maiden double ton, finishing unbeaten on 261, before handing over to Kent, who lost Ben Compton and Sam Northeast cheaply with all eyes on under-pressure England opener Crawley.
Crawley's Test spot is the subject of heavy scrutiny after an underwhelming Ashes and an average of just over 31 in 64 matches, meaning he could do with some big scores for Kent to shore up his position.
Having made nine and 20 at Durham last week, Crawley showed some patience in getting to 26 but he missed a drive off Harry Conway and had his off-stump knocked back before Kent closed on 119 for six.
Ben Duckett, Crawley's opening partner for England, was run out for one for Nottinghamshire before Joe Clarke's 136 lifted the county champions to a commanding 450-run lead over Glamorgan at Trent Bridge.
After Glamorgan were all out for 113 for a 166-run first-innings deficit, Duckett was dismissed by Sean Dickson's direct hit following a mix-up with Ben Slater, who made 70 in Notts' 284 for nine.
Ben McKinney has done his prospects of displacing either Crawley or Duckett no harm in Bristol, where he was eventually dismissed for 244 in Durham's 605 for five declared at Gloucestershire.
Kemar Roach and Ben Raine then cemented Durham's dominance with three wickets apiece as Gloucestershire went to stumps on 168 for eight, still trailing by 437.
Craig Overton's career-best 141 lifted Somerset to 348 in response to Essex's 149 at Chelmsford, where the hosts recovered from 45 for three in their second dig to close on 131 without further loss.
An unbroken 86 between Paul Walter (63 not out) and Matt Critchley (53no) helped Essex rally, although they still trail their opponents by 68 heading into the penultimate day.
Rishi Patel's unbeaten 133 and Australia opener Jake Weatherald's 96 helped Leicestershire fight back after Surrey had run up 520 at the Kia Oval, where the visitors closed on 350 for three.
Sonny Baker and Kyle Abbott claimed four wickets apiece to skittle Yorkshire for 177 before Hampshire went to stumps at Headingley on 93 for two and a lead of 167.
Rob Yates' unbeaten 75 underpinned Warwickshire's 154 for five and a lead of 217 after Sussex were all out for 204 at Hove.
Harry Came made 83 and Brooke Guest 60 not out in Derbyshire's 235 for three in response to Lancashire's 351 at Emirates Old Trafford, where James Anderson claimed one for 33 from 14 overs.
Fifties for openers Daniel Lategan (65) and Jake Libby (79) helped Worcestershire to 200 for seven and a lead of 208 against Middlesex, who were earlier bowled out for 183 at Lord's.
