In baboons, the placenta remains in situ and functional with respect to the potential for aromatization after removal of the fetus (fetectomy). Fetectomy therefore was used to study effects of the fetus and estrogen on placental delivery. By term, serum estradiol levels in untreated, intact baboons had increased to 4 to 8 ng/ml, and fetoplacental delivery occurred on day 184 +/- 1 (mean +/- SE). Fetectomy at midgestation resulted in a nondetectable serum estradiol level and a marked decline in progesterone level; however, placentas were maintained in situ and were delivered on day 171 +/- 6. After fetectomy therefore the initiation of placental delivery and, presumably, myometrial contractility did not require an elevation in estrogen. Administration of estradiol (1 to 10 mg/day) to baboons after fetectomy resulted in normal serum estradiol concentrations, but placental delivery was prevented. When estrogen was discontinued on days 215 to 250, the serum estradiol level declined, and placental delivery occurred on day 262 +/- 18, a value greater than in intact baboons or untreated baboons after fetectomy (p less than 0.001). Thus estrogen prevented placental delivery in baboons after fetectomy.