Serum levels of ornithine carbamyl transferase (S-OCT), glutamic oxaloacetic transaminase and glutamic pyruvic transaminase were compared for 37 Reye’s syndrome patients with regard to variation with clinical stage and serum ammonia levels. In stage I patients, the mean S-OCT activities were greater and the serum ammonia levels lower than found with patients in the more advanced stages. Covariation of these two parameters was found only in the more advanced stages. No significant correlation with stage or serum ammonia levels was found for S-GOT or S-GPT activities. These observations are discussed in terms of their relevance to reports of an early transient decrease of hepatic OCT activity in Reye’s syndrome.