The type of dietary fat affects the action of insulin by changes induced in the fatty acid composition of cell membranes. Little is known, however, about the effects of dietary fatty acids on insulin secretion or the possible relation between the fatty acid composition of the membrane phospholipids and insulin secretion. We therefore studied the effects of dietary fatty acids on insulin secretion stimulated by glucose, forskolin and arginine, and on the insulin content of isolated pancreatic islets, as well as on the fatty acid composition of muscle phospholipids, which were used as markers of the diet-induced modifications in the cell membranes. Five groups of rats were fed for one month with diets varying only in their fat composition: olive oil, sunflower oil, soybean oil, fish oil and palmitic acid (16:0) + soybean oil (SAT). The SAT group had higher insulin secretion, independently of the secretagogue used. No significant differences were found in insulin content between the groups. The dietary fatty acids modified the fatty acid composition of the muscle phospholipids, both in endogenously synthesized fatty acids and in those which were unable to be synthesized by the organism. No statistically significant relation was found between insulin secretion and the content of certain fatty acids in the muscle phospholipids.