Cellular insulin resistance is a common feature of the diabetic and obese state. To determine the effect of dietary fat and the insulin resistant state of diabetes on adipose tissue composition, control and streptozotocin-induced diabetic rats were fed four diets differing in fat content (10 percent w/w or 20% w/w) and polyunsaturated to saturated fatty acid (P/S) ratios (0.2 or 2.0) for 6 weeks. At 3 weeks diabetes was induced in half the animals in each diet group. Increasing the fat content and P/S ratio of the diet increased the content of polyunsaturated fatty acids and decreased the contents of monounsaturated and omega-3 fatty acids. The higher level of C-18:2(6) and the lower levels of C20:4(6) and monounsaturated fatty acids observed in diabetic animals is consistent with altered desaturase enzyme activity. Diet and diabetes induced compositional changes in essential and non-essential fatty acids in the adipose tissue may alter the total body pools of available fatty acids for the synthesis of other lipids such as phospholipids.