In the present study we have therefore addressed the issue of competitive interactions of dietary n-6 and n-3 EFA on the development of obesity in ob/ob mice. The effect of dietary EFA manipulation on PG synthesis was assessed by the generation of thromboxane (TX) A, by platelets in freshly clotted blood. In order to control accurately the EFA intake of the ln/ln and ob/ob mice, semi-synthetic diets were given in which the fat component was an oil, the EFA composition of which was primarily n-6 or n-3 fatty acids. These oils were evening primrose (Oenothera biennis) oil (720 g 18: 2n-6 and 90 g y-linolenic acid (18: 3n-6)/kg oil; EPO), and cod liver oil (80 g eicosapentaenoic acid (20: 5n-3) and 80 g docosahexaenoic acid (22: 6n-3)/kg oil; CLO). Our present results support the previous hypothesis that an imbalance in metabolism of n-6 and n-3 EFA exists in ob/ob mice (Cunnane et al. 1985). Supplemental n-3 EFA partly offset this abnormality whereas supplemental n-6 EFA contributed to it. Significantly lower production of TXB, by platelets in clotting blood from the ob/ob mice fed on supplemental n-3 EFA was also observed.