In several laboratories the determination of serum ornithine transcarbamylase activity (S-OTC) * has been used as a liver test. In the present study normal S-OTC values were established from results obtained from 222 individuals. Higher enzyme activities were found in persons older than 25 years than in those below this age. No differences in S-OTC between the two sexes were observed. The frequency distribution of S-OTC was asymetrical. No diurnal S-OTC variation occured. Hard physical work or a single heavy dose of alcohol did not influence the S-OTC level.