A soluble PRL receptor has been identified in the 100,000 X g supernatant from homogenates of lungs and livers of male and female rats treated with either estradiol (E2; 2 mg kg-1 day-1 for 7 days, sc) or ovine PRL (oPRL; 0.1 mg kg-1 day-1 for 14 days, sc). Fifty percent of the total PRL-binding activity in the liver homogenate of E2-treated male rats was found in the supernatant fraction, and only 12% in intact female rats. The soluble PRL receptor has the same specificity, protein dependence, and binding affinity (Ka = 2.8 X 10(9) M-1) characteristics as the membrane-bound receptor. A sheep anti-PRL-receptor antiserum specifically inhibited the binding of [125I]iodo-oPRL to the soluble PRL receptor. Column chromatography on Sepharose 6B revealed a single peak of [125I]iodo-oPRL-receptor complex from liver of E2-treated rats, having a mol wt of 340,000, whereas the 100,000 X g supernatant from lungs and livers of oPRL-treated rats revealed two specific peaks of [125I]iodo-oPRL complex with mol wts of 340,000 (A) and 165,000 (B), respectively. Peak A represented 25% and 27% and peak B, 35% and 49% of the total column radioactivity for liver and lung 100,000 X g supernatant fraction, respectively. Peak B coeluted with a rabbit anti-oPRL antiserum, suggesting that it is a PRL antibody. Anti-PRL-receptor antibody reduced the radioactivity associated with peak A but not peak B. Heat inactivation at 60 C (30 min) resulted in a complete loss of binding in peak A without affecting peak B. The results indicate that the soluble PRL-binding sites, increased in rat lung and liver after treatment with oPRL or E2, may represent an intermediate step in new receptor synthesis before incorporation into the membrane.