Others have shown that during perfusion of human placentae estrogen formation occurs less readily from androst-5-en-17-one-3β-yl sulfate (DHAS) than from 3β-hydroxyandrost-5-en-17-one (DHA) or androstenedione, indicating that sulfatase (S) activity may be rate limiting. The S, 3β-hydroxysteroid dehydrogenase-4,5-isomerase (D) and aromatase (A) activities of 10,000 g supernatant of placental homogenates were determined kinetically. Activities (nmole product formed/min/g protein) differed greatly and invariably S >D >A: e.g., S-705, D-46, A-14 (spontaneous term delivery); S-589, D-113, A-18 (term cesarean section); S-173, D-34, A-5 (hysterectomy, 12th week of gestation). Initial rates of estrogen formation from DHAS, DHA and androstenedione by placental microsomes, incubated with an NADPH generating system in air, were similar, confirming that the aromatase and not the sulfatase is rate limiting in. vitro. Hydrolysis of DHAS by S was inhibited by excess substrate, pregn-5-en-20-one-3β-yl sulfate, estrone sulfate, estrogens and C19 intermediates in their synthesis, progesterone, 3β-hydroxypregn-5-en-20-one and corticoids at 5×10−5m. Inhibitory effects were cumulative. It is suggested that inhibition of S by cord blood and placental steroids render it rate limiting during perfusions and provide a mechanism for control of placental estrogen synthesis in vivo.