Placental hypertrophy was induced in pregnant rats by daily treatment with estrone (0.5 .mu.g) and progesterone (4 mg) from day 3 through 19 combined with ovariectomy on day 12 of pregnancy. The ratio of basal zone to whole placenta was 47% by weight in these enlarged placentae on day 20 of pregnancy but only 34% in normal placentae. In each case, the basal zone tissue was homogenized, and the 10,000 .times. g supernatant fraction was used for in vitro incubation with [7.alpha.-3H]pregnenolone as the added substrate. The normal placental tissue synthesized more progesterone and testosterone but less androstenedione and 5.alpha.-pregnane-3,20-dione than the giant placentae. Utilization of the pregnenolone substrate by the giant placentae was subnormal. Addition of 200 IU of human chorionic gonadotropin (hCG) in vitro to normal basal zone placentae sharply increased production of progesterone, 17.alpha.-hydroxyprogesterone, androstenedione and 5.alpha.-pregnane-3,20-dione. The giant placentae failed to respond to hCG. Steroid-induced hypertrophied placentae in rats do not participate in a compensatory mechanism for steroid hormone production toward pregnancy maintenance.