The purpose of the present study was to investigate the effect of a concentrated preparation (EPA 30) containing eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3) on the limiting desaturation steps of the polyunsaturated fatty acid biosynthesis in spontaneously hypertensive rats (SHR). Adult SHR were divided into two groups: one group received a standard diet, and the experimental group the standard diet including 0.8% of EPA30 for 9 weeks. Blood pressure was measured at the end of the diets. The desaturase activities and fatty acid composition were determined in isolated hepatocytes. The blood pressure did not decrease in the experimental group. The desaturated products of the n-6 family (gamma-linolenic acid, 18:3 n-6 and arachidonic acid, 20:4 n-6) were lowered in the EPA30 group, when their respective substrates (18:2 n-6 and 20:3 n-6) were increased. EPA and DHA were higher in the experimental group delta 6 n-3, delta 6 n-6 and delta 5 n-6 desaturase activities were depressed approximately 20% in the EPA30 group. EPA30 being an active nutrient on the EFAs cascade, increasing the level of PG3 precursors and decreasing the level of PG2 precursors, favourable conditions have been established to reduce hypertension. The underlying mechanism related to the regulation of desaturase activities by these fatty nutrients remains to be elucidated.