Eighteen healthy volunteers consumed very low fat diets (less than 7% of daily energy) enriched with different sources of long chain (C20 and C22) polyunsaturated fatty acids (PUFA). Three diets provided 500 g/day of fish caught in the tropical waters of Australia (rich in arachidonic acid and docosahexaenoic acid), fish caught in the southern waters of Australia (rich in docosahexaenoic acid), or kangaroo meat (rich in linoleic and arachidonic acids). The fourth diet was vegetarian, similarly low in fat but containing no 20- and 22-carbon PUFA. An increase in the percentage of a particular C20 or C22 PUFA in the plasma phospholipid fraction in subjects consuming these low fat diets corresponded to the dietary PUFA composition. This study examined the effect of dietary modification of the level of arachidonic acid in plasma phospholipids on both traditional measures of platelet function and on cold-induced vasoconstriction. The cold pressor response, measured by venous occlusion plethysmography, was depressed in diets which elevated the levels of arachidonic acid in plasma lipids (kangaroo and tropical fish), enhanced after subjects consumed a diet which increased the levels of docosahexaenoic acid and eicosapentaenoic acid (southern fish diet), and was unchanged by the low fat vegetarian diet. There was no effect on bleeding time or platelet responsiveness.