Some derivatives of thieno[2,3-d]pyrimidin-4(3 H)-one A and their 1,2-dihydrogenated homologues B were synthesized. The study of their in vitro antiaggregating activity showed that the first compounds exhibited significant inhibiting power when aggregation was induced with ADP. Their reduction to derivatives of 1,2-dihydrothieno[2,3-d]pyrimidin-4(3 H)-one led to a new series of molecules possessing a large antiaggregant activity. When compared to that of acetylsalicylic acid under the same conditions, this activity was the same with collagen or arachidonic acid-induced aggregation, but greater when aggregation was induced by ADP. Serotonin release was also inhibited.