The effects of marine oil-enriched diets on the fatty acid compositio of lipids in guinea pig megakaryocytes (MK) and platelets were studied to obtain a better understanding of the mechanisms for changes in platelet fatty acid compositon and platelet function. Animals were fed 25%, 5% and 10% menhaden oil-enriched diets for up to 35 days. Platelets and MK were isolated and MK subpopulations at various stages of development were prepared. The diets did not cause a change in the cholesterol/phospholipid ratio in MK or platelets. The diets induced a dose related incorporation of eicosapentaenoic (20:5) and docosahexaenoic acid (22:6) and an associated decrease in linoleic acid (18:2) in both MK and platelets. However, there was a considerable greater depression of 20:4 in platelets than in MK. These changes were evident with 2% marine oil diets and maximal with 10% diets. Half maximal changes in fatty acid composition occurred after 3 days and maximal changes at 10 days after the initiation of the diets and no further changes occurred up to 35 days. Based on percent of total fatty acids in individual phospholipids, 20:5 had been primarily incorporated into phosphatidylethanolamine (PE) and phosphatidylinositol (PI) and 22: 6 into PE and hosphoatidylserine (PS) in both MK and platelets. 18:2 was decreased in all phospholipids. 20:4 was decreased only in PI in MK while 20:4 was decreased in PE, PI and PS in platelets. In animals on the 10% marine oil diet, more 20:5 and 22:6 were incorporated into mature than immature MK but the greatest amount of 20:5 and 22:6 had accumulated in platelets. Ingestion of marine oil-enriched diets did not cause thrombocytopenia or affect MK maturation based on the analysis of morphologic stage, ploidy or size. Marine oil-enriched diets caused a decrease in thromboxane synthesis in response to thrombin and calcium ionophore in platelets and MK at all stages of maturation. In platelet-rich plasma, collagen induced platelet aggregation, ATP secretion and thromboxane synthesis were decreased to a greater degree at 35 days than 10 days. Thus the study indicates that the ingestion of marine oil-enriched diets resulted in the compartmentalization of 20:5 and 22:6 in acidic phospholipids in mature MK and platelets. The observation that marine oil-enriched diet induced maximal changes in lipid composition in MK and platelets within 10 days but caused progressive inhibition of platelet function for up to 35 days indicates that as yet undefined membrane and cellular changes may occur at later time points.