Microsome-free preparations of rodent and human liver were shown to contain N-acetyl transferase from experiments using procainamide as substrate. These preparations then acetylated phenelzine from the quantitative transfer of radiolabeled acetate. This in vitro demonstration of phenelzine acetylation in rodent and human liver was corroborated by the finding of a negative correlation between excretion of phenelzine in urine and sulphadimidine acetylation in 27 patients.