Feeding young rats diets containing sunflowerseed oil (SSO) or palm oil (PO) induced several differences in the properties of the isolated epididymal fat cells: insulin stimulated deoxyglucose uptake 127% over the basal value in cells of the SSO group but only 47% in those of the PO group; the insulin concentration giving half maximal stimulation differing only slightly; insulin binding to the cells was higher in the SSO group; Scatchard analysis revealed that this was due to a significantly higher number of low-affinity binding sites, and the epididymal fat pad showed a concomitant change in the fatty acid pattern of the phospholipids, reflecting to a limited extent the differences in the composition of the diets. Neither the average diameters of the isolated fat cells, nor the serum insulin level at the time of sacrifice of the rats differed for the two types of dietary fat. These results indicate that a diet high in linoleic acid (SSO) induces a better response of fat cells to insulin than a diet high in saturated fatty acids (PO).