Prolactin (PRL) is essential for progesterone biosynthesis and luteal cell hypertrophy of the rat corpus luteum during pregnancy. Both the long and short form of the PRL receptor have been identified in the corpus luteum of pregnant rat. The long form has been shown to transduce PRL signal in other cells, whereas no information is available on the role of the short form, especially in the corpus luteum. In the present study, we have cloned a rat ovarian-specific phosphoprotein, PRAP (PRL Receptor Associated Protein), which has no significant homology to other known proteins. We have demonstrated that this protein is immunoprecipitated by anti-PRL receptor and anti-phosphotyrosine antibodies. To determine whether PRAP associates with either the long or the short form of the PRL receptor, fusion proteins with glutathione S-transferase containing the cytoplasmic domain of the long or short form of the PRL receptor were produced, purified, and incubated with luteal proteins. Our results indicate that PRAP preferentially binds to the short form of the PRL receptor. Thus, the long form and short forms of the PRL receptor may signal through distinct pathways. These data provide evidence for the involvement of a novel protein in PRL signal transduction and suggest that PRAP may contribute to the luteotropic effects of PRL on the corpus luteum during pregnancy.