The effects of streptozotocin diabetes on the activities of rat liver glycosyltransferase enzymes have been investigated. Liver microsomal fractions were prepared from rats that had been injected with streptozotocin (65 mg/kg, i.v.) 3 wk to 2 mo earlier. Preparations from diabetic rats had decreased activities of N-acetylglucosaminyl transferase compared with those of age-matched controls (0.98 +/- 0.11 nmol transferred per mg protein in 30 min versus 3.19 +/- 0.34 for controls, P less than 0.001). Galactosyltransferase activity was also lower in diabetic rat livers (1.48 +/- 0.26 nmol transferred per mg protein in 30 min versus 3.32 +/- 0.56 for controls, P less than 0.025). Sialytransferase activities were not significantly different between diabetic and control rat livers. There were no significant differences between the diabetic and control rat liver microsomes in the activities of UDP N-acetylglucosamine pyrophosphatase, UDP galactose pyrophosphatase, or CMP sialic acid phosphatase. The glycosidases, N-acetylglucosaminidase and galactosidase, had similar activities in the livers of both groups of rats. Sialidase activity could not be detected in microsomal preparations from either diabetic or control rat livers. These results are discussed in relation to our previously reported alterations in glycosyltransferase activities, and plasma membrane glycoprotein composition in the livers of rats made insulin-resistant by a carbohydrate-free, high-fat diet and to the observation of Carter and his colleagues (FEBS Lett. 1979; 104:389-92.) that streptozotocin diabetes alters the glycoprotein composition of rat liver plasma membranes.