Hales ton helps power Notts
Alex Hales hit 183 as Nottinghamshire piled up 397-7 on day one after being asked to bat by Warwickshire, for whom Jonathan Trott was making his first-class comeback.
England batsman Trott, who was forced home from last winter's Ashes tour by a stress-related condition and suffered a recurrence 10 weeks ago, claimed the wicket of opener Phil Jaques (77) with his occasional medium pace to end a second-wicket stand of 91.
Bat - in particular Hales' - continued to dominate ball for much of the day at Edgbaston; the Notts No 3 dispatched 25 fours and a six in his 224-ball effort before he became one of Boyd Rankin's three victims.
Hales, a T20 regular for England who has yet to play either ODI or Test cricket, now has two championship hundreds and three half-centuries this season.
Riki Wessels also contributed 53 but Warwickshire hit back with three late wickets.
At Taunton, Paul Horton's 19th first-class century guided Lancashire to 221-6 after they chose to bat against Somerset.
The opening batsman was out less than four overs before stumps for 140, lbw to Lewis Gregory (2-39).
Horton received little support from his Lancashire top-order colleagues with 18s from Stephen Croft and Jos Buttler - playing against his former county - the next highest scores on the card.
Gregory's fellow seamer Alfonso Thomas (2-27) also claimed two wickets.
Middlesex's top three all scored half-centuries after they had been sent in by Division One's bottom side Northamptonshire at Lord's.
Chris Rogers (86) and championship debutant Nick Gubbins (54) shared an opening stand of 128 and No 3 Dawid Malan finished the opening day on 72 not out in a total of 280-3.