PRL acts in the ovary to promote the maintenance and function of the corpus luteum. However, the cellular signals that induce these responses have not been clearly defined. In the present report we demonstrate that Stat 5, previously identified as a transcription factor activated by PRL in the mammary gland, is also activated by PRL in the ovaries of pseudopregnant rats. Intraperitoneal injection of PRL into pseudopregnant rats results in the tyrosine phosphorylation and nuclear translocation of Stat 5. This activated Stat 5 possesses DNA-binding activity for a sequence containing the PRL-inducible element. In addition, we report that luteinization of the PMSG-primed ovary by the administration of hCG is accompanied by an induction of Stat 5 protein.