Male Sprague-Dawley rats were fed diets with 10%, 30%, or 50% of energy derived from fat for two weeks. The fats used were beef tallow, olive oil, peanut oil and butter. Aortic prostacyclin (PGI2) production, platelet aggregation and thromboxane A2 (TXA2) production and plasma and aortic phospholipid (PL) content were measured. Butter- and beef tallow-feeding reduced aortic PGI2 production and collagen-induced TXA2 production in a dose-dependent manner as the level of fat in the diet increased. Neither olive oil nor peanut oil had any effect on aortic PGI2 production or collagen-induced TXA2 production. Butter-feeding also resulted in a decrease in collagen-induced platelet aggregation; however, none of the other fats had any effect on collagen-induced platelet aggregation. The observed decreases in aortic PGI2 and collagen-induced TXA2 production were paralleled by similar decreases in aortic and plasma PL arachidonic acid content and an increase in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Only the most saturated fats, butter and beef tallow, had significant inhibitory effects on prostanoid production and platelet aggregation.