Plafibride is an acyl derivative of morpholinomethylurea (MMU) with clofibric acid. Plafibride is hypolipemic, platelet aggregation inhibiting agent. Plafibride did not act on the arachidonic acid metabolism. Plafibride inhibited the activity of 3',5'-cyclic AMP-phosphodiesterase, which is one of the principal mechanisms of inhibition of platelet aggregation. In the rat, plafibride inhibited significantly the spontaneously formed circulating platelet aggregates. In vitro plafibride appeared as an effective antiaggregant agent although less powerful than morpholinomethylurea, one of its presumed metabolites. The most evident secondary effects of plafibride were: a certain sedation, as a light tranquilizing agent, a hypothermic effect when it was administered at high doses, a certain beta-blocking and antiarrhythmic activity probably due to its local anesthetic action. All the side effects appeared at high doses, much higher than the therapeutic ones.