Centuries by Matthew Pardoe and Moeen Ali put Worcestershire in charge on a bad first day for Glamorgan in their LV= County Championship Division Two meeting at New Road.
Mark Wallace's decision to bowl first on winning the toss backfired on the Welshmen as two left-handers shared in the home side's first double-century partnership in the competition for nearly three years.
Pardoe completed an impressive maiden hundred with 102 from 235 balls, and Moeen cemented his position as the leading scorer in Division Two by making an unbeaten 155 towards a total of 322 for three.
Already, Worcestershire are in far better shape than at Cardiff in April when Australian seamer Michael Hogan took seven wickets in Glamorgan's only victory in seven championship games so far.
Hogan has since become the leading wicket-taker in the division and it was his opening spell of 1-9 in seven overs that gave Worcestershire their toughest examination in the return meeting.
The 6ft 5in seamer cut one back into Daryl Mitchell's pads to claim the only success in the first two sessions, and after lunch he was as close as anyone to unseating Moeen when the batsman got away with an edge past the slips.
Once the shine had gone from the ball, the conditions favoured the batsmen and Moeen, in his first innings since his 26th birthday on Tuesday, was clearly determined to profit on a slow surface.
His second 50 included only three boundaries and two of these came from successive balls off Marcus North as he moved up to 99. A single in the Australian's next over took him to three figures from 169 balls.
For Pardoe, it was a new experience to move through the 90s and, after surviving one big lbw appeal, the Stourbridge-born academy product raised his hundred with leg-side blows for six and four off North.
This is shaping up to be the best of his three seasons in first-team cricket. Having scored fifties in two of the previous home championship games, he took the big step by hitting 13 fours and two sixes.
The next ball he faced proved to be the last. Propping forward, he edged to Jim Allenby, giving Dean Cosker some compensation after conceding 22 runs in two overs before lunch.
The second-wicket stand eventually produced 219 in 64 overs, the county's best for any wicket since Moeen put on 215 with James Cameron in a promotion-clinching win against Sussex in September 2010.
Glamorgan's struggle continued as Moeen and Thilan Samaraweera, who made 38, put on 90 before the Sri Lankan steered Allenby to backward point where Ben Wright dived to his right to hold a sharp chance.