Matt Pardoe and Moeen Ali put Worcestershire in charge on a bad first day for Glamorgan in their County Championship Division Two meeting at New Road.
Pardoe completed his maiden hundred with 102 from 235 balls, and Moeen cemented his position as the leading scorer in Division Two in making an unbeaten 155 towards a total of 322-3.
Glamorgan captain Mark Wallace's decision to bowl first on winning the toss backfired as two left-handers shared in the home side's first double century partnership in the competition for nearly three years.
Michael Hogan 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 50s in two of the previous home championship games, he took the big step after 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 and ending a second-wicket stand of 219 in 64 overs.
Glamorgan's struggle continued as Moeen and Thilan Samaraweera (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.