Serial estimations (87) of serum cystine aminopeptidase (CAP) in pregnant women (18) using diphenyl hydantoin, phenobarbital or diazepam revealed that the enzyme levels were below the average for normals in 89% of patients. The initially low CAP levels in women on diazepam were improved within 24 h after betamethasone administration. Subsequent studies in vitro confirmed that diphenyl hydantoin, phenobarbital, diazepam and acetyl salicylate markedly inhibited CAP levels in sera of women (72) with clinically uncomplicated pregnancy. No inhibition of CAP by any of the drugs tested was observed in sera from nonpregnant women (8). However, nonpregnant women receiving medroxyprogesterone acetate injections showed CAP inhibition by various drugs, suggesting some changes in the enzyme moiety under the influence of the hormone.