This work was undertaken to determine variations in the 125I-labelled ovine prolactin (oPRL) binding in rat liver and mammary gland membranes, and to study the molecular forms of prolactin receptor in different physiological situations. Prolactin binding was determined using 125I-labelled oPRL in the 100,000 g pellet. 125I-Labelled oPRL was cross-linked to receptors in membranes from rat liver and mammary gland and subjected to SDS-PAGE under reducing conditions, followed by autoradiography of dried gels. In the mammary gland, the specific binding of oPRL to membranes, expressed as mean +/- S.E.M. fmol/mg protein increased from 1.36 +/- 0.24 on the day of dioestrus to 3.26 +/- 0.60 on the day of oestrus. It remained very low during pregnancy but increased during lactation to reach 4.78 +/- 0.99. In the liver, the specific binding of oPRL to membranes was higher than in the mammary gland on the days of dioestrus 1, dioestrus 2 and oestrus, but not on the day of pro-oestrus. It increased until day 14 of pregnancy when the specific binding of 125I-labelled oPRL was 17.01 +/- 0.30. Cross-linking revealed different molecular forms in the mammary glands and the liver. In the mammary gland we observed four prolactin-binding forms of 80, 50, 40 and 16 kDa, all of which were specific for prolactin. The 80, 50 and 40 kDa forms were also able to bind to a Concanavalin A-Sepharose column, indicating that these binding forms were glycosylated while the smaller one (16 kDa) appeared to be unglycosylated.(ABSTRACT TRUNCATED AT 250 WORDS)