The primary structure of the rat liver prolactin receptor has been deduced from a single complementary DNA clone. The sequence begins with a putative 19 amino acid signal peptide followed by the 291 amino acid receptor that includes a single 24 amino acid transmembrane segment. In spite of the fact that the prolactin receptor has a much shorter cytoplasmic region than the growth hormone receptor, there is strong localized sequence identity between these two receptors in both the extracellular and cytoplasmic domains, suggesting that the two receptors originated from a common ancestor.