The binding of prolactin (PRL) to the plasma membranes of bovine and human ovaries was investigated using both homologous and heterologous 125I-prolactin. Saturation and Scatchard analysis demonstrated that human prolactin binds to human ovarian membranes with a Kd of 2 x 10(-10) M; to bovine ovarian membranes with a Kd of 1.9 x 10(-10) M; and to bovine corpora lutea membranes with a Kd of 1.9 x 10(-10) M. The concentrations of binding sites in bovine and human ovaries were 2.9 x 10(-15) moles/mg of protein and 2.0 x 10(-15) moles/mg of protein, respectively. The number of bindings sites in the bovine corpora lutea was 1.5 x 15(-15) moles/mg of protein. Specificity studies with bovine PRL, ovine PRL, human luteinizing hormone, human follicle-stimulating hormone, and bovine growth hormone showed this binding to be specific. Comparison of binding of PRL to membranes of other target and nontarget tissues suggests that the ovary is one of the primary target tissues. These data suggest that prolactin plays a role in the ovarian cycle.