The effect of diabetes in rats on lipid composition and order of synaptosomal membranes (SM) was determined in streptozotocin-induced diabetic rats after 6 weeks of chronic hyperglycemia. The cholesterol content was slightly, but not significantly, higher in diabetic SM (0.287 +/- 0.042 vs. 0.209 +/- 0.061 mumol/mg protein). The phospholipid concentration in diabetic SM was significantly increased (0.515 +/- 0.042 vs. 0.305 +/- 0.041 mumol/mg protein; P less than 0.005). Neither the molar ratios of cholesterol to phospholipids in the SM nor the fatty acid composition of the SM was significantly altered with diabetes. Diabetes did not affect membrane order or the thermotropic transition temperature of the SM as determined fluorometrically. On the other hand, the SM of diabetic rats had significantly increased concentration of lipid peroxidation products, namely conjugated dienes (the calculated O.D./mumol phospholipids was 11.56 +/- 1.83 in controls and 19.95 +/- 4.1 in diabetic rats P less than 0.01). Despite the accumulation of lipid peroxidation byproducts in SM of diabetic rats the overall membrane order and the cholesterol to phospholipid molar ratio do not appear to be significantly altered.