Removal of fetuses at day 14 of gestation (Ftx14) in the pregnant rat leads to a marked suppression of serum levels of rat placental lactogen (rPL-II). One might attribute this to compromised placental growth in the absence of a fetus. However, if ovariectomy and fetectomy (Ftx14 Ovx14) are carried out at the same time, a great increase in serum rPL-II levels is seen. This occurs despite a significant decrease in placental weight. When Ftx14 was performed on day 14 and Ovx was delayed 1, 2, or 3 days, the expected large increase in serum rPL-II was progressively attenuated compared to that seen when Ftx and Ovx were carried out simultaneously. Daily administration of 17 beta-estradiol (4 micrograms/rat X day) to Ftx14 Ovx14 pregnant rats resulted in a significant suppression of rPL-II and elevation of rPRL levels, a reversal of what is seen for these hormones in untreated Ftx14 Ovx14 animals. To test whether 17 beta-estradiol was acting through rat PRL (rPRL), serum levels of rPRL were elevated in Ftx13 Ovx13 animals with pimozide (0.6 mg/kg), a dopamine receptor blocker. There was no effect of this treatment on rPL-II levels. In late pregnancy (day 17) serum rPL-II levels remained high after removal of half the fetuses (1/2 Ftx), compared to the rapid fall in pregnant rats in which all fetuses were removed (Ftx17). Serum levels were also elevated in 1/2 Ftx animals compared to those in which half of the fetuses and placentas were removed by hemi-hysterectomy, suggesting that the increase in rPL-II levels in 1/2 Ftx animals was due to a stimulatory effect of the remaining fetuses on all of the placentas. These results indicate that the presence of the fetus is necessary for the normally observed increase in rPL-II levels in late pregnancy. In conclusion, fetal stimulators and ovarian inhibitors influence rPL-II secretion.