Viral hepatitis in chimpanzees produces negligible symptomatology, and serum aminotransferase changes may be minimal. To maximize the predictive value of these determinations, which are the only serum indicators available for non-A non-B (NANB) hepatitis infection, normal ranges for aspartate and alanine aminotransferases (AST, ALT) were examined and categorized according to age and sex. Males were found to have higher values than females, and adults higher values than juveniles. The kinetic method used and the values obtained are described. Differences in methodologies and reporting units are discussed.