Previous studies from this laboratory indicate that ovine placental lactogen (oPL) and ovine growth hormone (oGH) stimulate amino acid transport in diaphragms of postnatal rats with equal potencies. However, in studies reported here using diaphragms from fetal rats on day 20 of gestation, oPL (2,5 and 20 micrograms/ml) stimulated a dose-dependent increase in amino acid uptake, while oGH (5,20 and 100 micrograms/ml) and rat growth hormone (rGH, 2 and 40 micrograms/ml) were without effect. The effect of oPL on fetal AIB transport was neither enhanced nor antagonized by oGH (100 microgram/ml). The magnitude of stimulation of AIB transport by oPL was comparable to that observed with insulin (100 and 1000 microU/ml). Human placental lactogen (hPL) and ovine prolactin (oPRL) had no effect on fetal AIB transport. Since oPL is present in high concentrations in fetal blood, these studies suggest that oPL may have a direct role in the regulation of fetal amino acid and protein metabolism, that oPL and oGH may bind to different receptors in fetal rat tissues, and that oPL may function as a "fetal growth hormone".