In the present study we measured the bleeding times in fourteen Aborigines (10 diabetic, 4 non-diabetic) before and after 2 weeks on a diet of tropical seafood (rich in both arachidonic acid and the omega 3 PUFA), followed by 3 weeks on a diet in which kangaroo and freshwater fish (linoleic and arachidonic acid-rich) were the major fat sources. Both diets were very low in fat. Bleeding times increased in all subjects after the 2 weeks of tropical seafood and continued to rise on the mixed diet. The increase over 5 weeks from 4.1 +/- 0.4 to 5.9 +/- 0.4 min was highly significant (p less than 0.01). Due to the extreme isolation of the study location it was only possible to measure the plasma fatty acid composition at the beginning and end of the study. The concentration of arachidonic acid in the plasma lipids doubled whereas that of linoleic acid was almost halved, despite the fact that the diet in the second part of the study contained considerably more linoleic than arachidonic acid. That there appeared to be preferential incorporation of arachidonic acid into the plasma lipids is further supported by the observation that the rise in arachidonic acid in the cholesterol ester and phospholipid fractions was almost exactly counter-balanced by the fall in linoleic acid. In conclusion, the present study demonstrated a rise in bleeding time associated with an increased concentration of arachidonic acid and decreased concentration of linoleic acid in plasma lipids, and suggests that the mechanism by which diet modulates haemostatic function may be more complex than currently assumed.