Diaz came on stage first at the Bell Centre, accompanied by younger brother and fellow fighter Nate. Nick went old-school, representing his California home town with a Stockton hoodie. Underneath Diaz, who fought in Pride, wore a Pride Fighting Championship T-shirt.
The 29-year-old was stone-faced as the crowd at the Bell Centre booed.
Diaz (27-8 with one no contest) weighed in at 169 pounds.
A business-like St-Pierre came next, bounding up the stairs to the stage to cheers with head coach Firas Zahabi behind him. St-Pierre (23-2) weighed in at 170.
The 31-year-old champion wasted no time heading to Diaz and the side of the stage for the traditional pose. Diaz then shoved an elbow GSP's way and UFC president Dana White stepped in between them.
St-Pierre moved back, with a trace of a smile. Both fighters then threw their arms in the air.
"There's going to be a fight and we'll going to see what's going to happen," Diaz said to boos, before thanking Stockton and his support team.
"I can talk as much as I want but tomorrow I'm going to walk the walk," said St-Pierre. "That's all I can say."
The two face off Saturday night at the Bell Centre.
There was little drama in the rest of the weigh-in although English lightweight Colin (The Freakshow) Fletcher may have scared children with his skeleton-like voodoo mask and fanged mouthpiece. The six-foot-two tattooed Englishman is an eyeful without the props.
Montreal's Mike Ricci, his opponent, did not seemed bothered by the party favours.
And six-foot-one bantamweight George Roop, looking like a scarecrow at 135 pounds, tried to intimidate five-foot-seven Reuben Duran by sticking his head over his opponent's.
Montreal's Patrick (The Predator) Cote and Americans Jake (The Juggernaut) Ellenberger and Jonny (Bigg Rigg) Hendricks wore Canadiens jerseys to the scales.
American lightweight Daron (Detroit Superstar) Cruickshank showed his national colours by wearing Stars and Stripes underwear.