Specific binding sites for ovine placental lactogen (oPL) were detected in the 100,000 .times. g pellet obtained from tissues derived from non-pregnant and pregnant ewes and from fetal tissues. Specific binding of ovine [125I]iodo-PL (percentage per milligram of protein) was found in non-pregnant ewe liver (28%), adipose tissue (20%), ovary (10%), corpus luteum (6%), uterus (5%) and fetal liver (5%). The specific binding of oPL was less than 3% in the following maternal tissues: mammary gland, adrenal, kidney, spleen, pancreas, lung, heart, skeletal muscle, brain (cortex) and uterus near term (130-135 days'' gestation). Less than 3% specific binding was found also in several fetal tissues including adipose tissue, adrenal, kidney, spleen, pancreas, lung, heart, skeletal muscle, brain (cortex) and placenta near term (both fetal cotyledons and maternal caruncles). The binding of oPL to maternal liver, adipose tissue, ovary, corpus luteum and fetal liver was inhibited by ovine GH [growth hormone] but not by ovine PRL [prolactin] or ovine LH [luteinizing hormone]. In the radioreceptor assay for oPL using liver (100,000 .times. g pellets) from the ewe, oving [125I[iodo-PL was inhibited by several GH preparations and by human placental lactogen. PRL preparations from several species as well as other pituitary hormones did not compete for the binding sites. Conversely, oPL only slightly inhibited the binding of ovine [125I]iodo-PRL in the radioreceptor assay for oPRL using pregnant ewe uterus. oPL may act more like a GH than a PRL, and may mediate important metabolic effects during the greater part of pregnancy in the ewe.