Piglets received a basal diet to which either lard fat (L) or a mixture of lard fat and mackerel oil (ML) was added. After 2 months a constrictor was placed around the left descending coronary artery (LADCA) and the dietary regimen continued for 2 more months. During the dietary period plasma thromboxane B2 levels, thromboxane B2/6-keto-PGF1 alpha ratio, platelet membrane 20:4 n-6 content and ADP-induced platelet aggregation were all lower in ML than in L. Morphological examination at the site of the LADCA stenosis showed a higher encroachment due to intimal proliferation in L. It is postulated that alterations in eicosanoid synthesis indirectly affected the rate of intimal proliferation.