19-Nor-deoxycorticosterone (19-nor-DOC) is a mineralocorticoid that is increased in some forms of experimental and human hypertension. The pivotal step in 19-nor-DOC biosynthesis is adrenal P450 19-hydroxylase, but this enzyme has not been clearly distinguished from P450 11β/18-hydroxylase. This study attempted to specifically inhibit adrenal 19-hydroxylation of deoxycorticosterone (DOC) using a suicide aromatase inhibitor, 19-acetylenic androstenedione (19-AA). Purified bovine P450 11β/18/19-hydroxylase was incubated with excess substrate DOC, adrenodoxin, and adrenodoxin reductase in the presence of increasing doses of the inhibitor, 19AA. 11β-, 18-, and 19-hydroxylation were measured by quantification of corticosterone, 18-OH-DOC, and 19-OH-DOC respectively. Measurements of these products demonstrated that 11β- and 18-hydroxylation was not inhibited whereas 19-hydroxylation was inhibited as manifested by decreased 19-OH-DOC formation (p <.05). The IC50 of 19-AA was approximately 10-12M. The specific inhibition of 19-hydroxylation suggests that the 19-hydroxylase may be an enzyme distinct from the P450 11β/18-hydroxylase. This further suggests that 19-nor-DOC biosynthesis may be under independent regulation and may be amenable to specific in vivo inhibition.