1. The nipecotamide alpha,alpha'-bis[3-(N,N-diethylcarbamoyl)piperidino]-p-xylene (A-1) is a platelet aggregation inhibitor. The meso diastereomer A-1c is superior in potency and duration to the synthetic diastereomeric mixture consisting of the R,R-, S,S-, and R,S- (meso) isomers in inhibiting collagen-induced platelet aggregation ex vivo in the dog. 2. A-1c also is more potent and longer acting than A-1 in protecting mice from collagen+epinephrine-induced thromboembolic death. 3. The mechanism of antiplatelet action of this compound appears to be related to its ability to prevent agonist-induced inhibition of platelet cyclic adenosine monophosphate (cAMP) levels.