The rat liver prolactin receptor has been purified to homogeneity, and partial amino acid sequences have been obtained. The structure of the receptor has been deduced from a single complementary DNA clone. The mature protein of 291 amino acids has a relatively long extracellular region, a single transmembrane segment, and a short (57 amino acids) cytoplasmic domain. With the rat cDNA used as a probe, the prolactin receptor in rabbit mammary gland and human hepatoma cells has also been isolated. These tissues contain a second, longer form of the receptor (592 and 598 amino acids, respectively). Both the short and long forms of the prolactin receptor show regions of strong sequence identity with the human and rabbit growth hormone receptors, suggesting that the prolactin and growth hormone receptors originate from a common ancestor.