A fish oil diet has been shown to lower systemic blood pressure in man and alter vascular reactivity to exogenous noradrenaline in man and rats. The effects of a fish oil diet on the vascular reactivity of the pulmonary circulation have not been assessed. The effect of dietary lipid composition on pulmonary artery pressure and vascular reactivity of 30 adult male Sprague-Dawley rats (250-300 g) was assessed in vivo (n = 9) and in the isolated perfused lung (n = 21). Isocaloric diets containing 20% lipid (% by weight) as fish oil, corn oil, or lard were given in unlimited quantities to the rats. After one month of the diet, pulmonary vascular reactivity to angiotensin II (0.15 micrograms) and hypoxia (fractional inspired oxygen 0.025) were assessed in the isolated perfused rat lung. The haemodynamic effects of the diet on resting normoxic pulmonary artery pressure (in vivo) were assessed in chloralose anaesthetised rats. The fish oil diet appreciably increased the content of eicosapentaenoic acid (C20:5) and docosahexaenoic acid (C22:6) in lung phospholipids but had no effect on pulmonary vascular reactivity. The arachidonic acid (C20:4) content of the lung was considerably decreased. Platelet counts were lower in the rats fed fish oil. The study indicates that a fish oil diet does not alter acute intrinsic pulmonary vascular reactivity despite pronounced changes in lung phospholipid fatty acid profile.