In this report, we describe the isolation and characterization of a full length cDNA clone for rat prolactin-like protein C (PLP-C) and describe the expression of PLP-C mRNA in the developing rat placenta. Nucleotide sequence analysis of the PLP-C cDNA clone predicted a mature protein of 238 amino acids, including a 30-amino acid signal sequence. The predicted PLP-C amino acid sequence contains seven cysteine residues, three tryptophan residues, and two putative N-linked glycosylation sites. Six of the cysteine residues in PLP-C are located in positions homologous to the cysteines of pituitary prolactin (PRL). Additional sequence similarities with pituitary PRL and other members of the rat placental PRL family are evident. The PLP-C gene was localized to rat chromosome 17. Northern blot analysis showed that the PLP-C cDNA clone specifically hybridized to a 1.0-kilobase mRNA. PLP-C mRNA was first detectable between days 13 and 14 of gestation, peaked by day 18 of gestation, and remained elevated until term. In situ hybridization analysis indicated that PLP-C mRNA was specifically expressed by spongiotrophoblast cells and some trophoblast giant cells in the junctional zone region of rat chorioallantoic placenta.