A hypolipidemic agent, pentaerythritol tetranicotinate (niceritrol) yields nicotinic acid upon hydrolysis in vivo, but niceritrol is hardly soluble in distilled water, so that the effects of nicotinic acid on platelet aggregation in vitro were studied. Nicotinic acid inhibited in vitro platelet aggregation induced by ADP, collagen and adrenaline. Twenty patients (61.4 +/- 2.4 years (mean +/- S.E.)) with ischemic heart disease, cerebral infarction, transient cerebral ischemic attack and hypercholesterolemia were given niceritrol orally at 750 mg per day for 8 weeks. Significant decreases in ADP-, collagen- and adrenaline-induced platelet aggregation were observed at 4 and 8 weeks after niceritrol treatment. There was a significant correlation between the rates of changes in platelet aggregation and those in plasma total cholesterol or plasma LDL-cholesterol before treatment and 8 weeks following treatment. The results indicate that niceritrol has inhibitory effects on platelet aggregation not only caused by its direct action on platelet but mediated by its secondary action due to decrease in blood lipids.