To determine whether changes in the relative biological potencies of ovine placental lactogen (oPL) and ovine growth hormone (oGH) during development derive from ontogenetic changes in the binding of these hormones to hepatic receptors, we have compared the binding of 125I-oPL and 125I-oGH to hepatic membranes from fetal lambs and pregnant sheep at mid- and late gestation and from postnatal sheep at 1 day to 7 mo of age. Specific high-affinity 125I-oPL binding sites in ovine fetal liver were detected as early as day 70 of gestation (term = 145 days), and the number of fetal 125I-oPL binding sites increased progressively throughout the latter half of gestation, reaching a maximum (11.2 fmol/mg protein) at 3-7 days before parturition. The potency of oPL (Kd 0.27 nM) in competing for 125I-oPL binding sites was 90 and 1,300 times greater than that of oGH and ovine prolactin, respectively. Although the number of fetal 125I-oPL binding sites increased throughout pregnancy, there was little or no specific binding of 125I-oGH noted in the fetus. Treatment of fetal liver membranes with 4 M MgCl2 did not enhance the subsequent specific binding of 125I-oGH, suggesting that the low specific binding of oGH did not result from occupation of hepatic receptors by endogenous circulating oPL or oGH. In contrast, MgCL2 treatment markedly increased the apparent number of fetal 125I-oPL binding sites, suggesting that oPL receptors in fetal liver are partly saturated in vivo by oPL.(ABSTRACT TRUNCATED AT 250 WORDS)