For at least 3-4 weeks, young male Wistar rats were fed semisynthetic diets containing sunflowerseed oil, palm oil, olive oil, linseed oil, cocoa butter and coconut oil as dietary fat. The type of dietary fat had little effect on body weight, epididymal fat pad weight and on the diameter of fat cells isolated from the epididymal fat pads, but the rats fed the linseed oil-containing diet had a lower epididymal fat pad weight. The fatty acid composition of the triacylglycerol fractions of the fat pads correlated well with those of the dietary fats. The correlations with the fatty acid composition of the phospholipids of the fat pads were less pronounced. High responses to insulin in the epididymal fat cells were obtained with sunflowerseed oil, linseed oil and olive oil, whereas low responses were found for cocoa butter, palm oil or coconut oil. Rather than the amount of polyunsaturated fatty acids or the ratio of polyunsaturated to saturated fatty acids, the amount of saturated fatty acids with 12, 14 or 16 carbon atoms appeared to be the most important parameter in determining the maximal insulin response. A negative correlation was found between the amount of saturated fatty acids in the diet and the extent of insulin response. The modulating effects of the dietary type of fat on the response to insulin cannot be fully explained by changes in the number of insulin receptors on the fat cell surface as determined by insulin binding but must, at least partially, be ascribed to postreceptor effects.