Suspensions of isolated rat hepatocytes incubated in the presence of the diabetogenic agent alloxan exhibit time- and concentration-dependent damage. At concentrations of 3.5 mM and above, alloxan caused an increase in lactate dehydrogenase (LDH), glutamate-pyruvate transaminase (GPT) and intracellular potassium (K+) leakage, all of which are indices of plasma membrane damage, and decreased the intracellular reduced glutathione content (GSH) of the cells. Preincubation (10 min) in D-glucose (50 or 100 mM, but not 10 mM) partially protected the hepatocytes from LDH, GPT and K+ leakage and the decrease in GSH produced by alloxan (7 mM) during a 60-min incubation period. Other sugars (D-galactose, 2-deoxy-D-glucose, D-fructose, D-mannoheptulose and D-mannitol) were also found to protect hepatocytes against damage caused by alloxan. D-Fructose was found to be the most potent protective sugar. These results indicate that alloxan is not selectively toxic to the pancreatic beta-cell and that sugars can protect against alloxan-induced cytotoxicity in hepatocytes.