Functional corpora lutea were induced in 28 day old, female rats with 4 iu of pregnant mare's serum, followed 72 h later with cervical stimulation. PGF2α (3 mg/kg) and ergocryptine (2 mg/kg) were administered (sc) 7 days following cervical stimulation. Prolactin (NIH-S11; 2 iu) was administered on the same day 4 h before, and 3 and 15 h after drug treatment. Animals were sacrificed at either 2, 8 or 24 h following treatment with PGF2α or ergocryptine. Serum progesterone was significantly depressed (P2α treatment from the 0 h concentration of 40.5 ± 12.4 ng/ml. Serum 20α-ol was unchanged at 2 h (11.2 ± 5.6 ng/ml), but was significantly elevated (P2α treatment. The LH receptor was decreased 16, 30 and 72 percent at each respective time period but the change was not significant 2 h after PGF2α treatment. Simultaneous treatment of the animals with prolactin blocked the effect of PGF2α on serum progesterone and the LH receptor when animals were sacrificed 24 h after PGF2α treatment.