In order to investigate the effect of fish oil on intimal proliferation of coronary arteries with a fixed stenosis, piglets were fed a diet to which either 9% (w/w) lard fat (L, n = 8) or 4.5% (w/w) lard fat and 4.5% (w/w) mackerel oil (ML, n = 8) was added for 4 months. The stenosis was applied by implanting a Teflon constrictor around the left anterior descending coronary artery (LADCA) after the animals had been on their diet for two months. Levels of plasma cholesterol and triglycerides did not change in L, but decreased in ML, while ADP-induced platelet aggregation in whole blood was higher in L than in ML. At the end of the dietary period the animals were anaesthetized and myocardial perfusion and systolic segment length shortening (SLS) were measured while the hearts were paced at 160 pulses min-1. Transmural blood flow to the non-LADCA nourished zones were normal and similar for L and ML, but impaired in the LADCA-perfused myocardium. In particular the subendocardial layers of L were hypoperfused resulting in a larger loss of SLS in L than in ML. A significantly higher encroachment due to intimal proliferation was found in L (62 +/- 7%) than in ML (11 +/- 4%) at the site of the constrictor while this was less than 2% adjacent to the constrictor. We conclude that the presence of the constrictor causes intimal proliferation and that the fish oil-induced changes in platelet aggregation may play a role in determining the extent to which this occurs.