[1β-3H], [1α,2α-3H] and [1β, 2β-3H] 4-Hydroxyandrostenedione (4-OH-A) were synthesized to study the mechanism of inhibition of aro-matase by 4-OH-A. Incubations of [1β-3H] and [1β,2β-3H] 4-OH-A with placental microsomes in the presence of NADPH showed very little loss of tritium, with aromatization of 4-OH-A ranging from 0.3 to 0.6 percent. No loss of tritium was observed in the absence of NADPH. The extent of covalent binding of 4-OH-A to microsomal proteins was higher with incubations in the absence of NADPH than with those in the presence of NADPH. These results are discussed in light of what has been proposed for the mechanism of androgen aromatization.