Prolactin has direct effects on the CNS. The highest concentration of prolactin receptors resides within the choroid plexus where they probably function to transport prolactin from blood into CSF. Another member of the lactogen family of hormones, placental lactogen (PL), also affects CNS activity and may similarly employ the cerebroventricular system as an intermediary. In order to determine whether the choroid plexus was a PL target tissue, in vitro autoradiography was used to identify specific PL binding sites in the choroid plexus of pregnant New Zealand White rabbits. Frozen brain sections were incubated in a medium containing 125I human PL (hPL) alone (total binding) or with a 500-fold excess of unlabelled hPL (nonspecific binding). The specificity of the binding was assessed with unlabelled human growth hormone (hGH) and ovine luteinising hormone (oLH). An intense autoradiographic reaction occurred over the choroid plexus of tissue sections incubated with 125I hPL alone. Excess unlabelled hPL and hGH, which is lactogenic in the rabbit, caused a significant reduction (P < 0.001) in the binding of radiolabelled hPL to the choroid plexus. In contrast, unlabelled oLH had no effect on radiolabelled hPL binding to this tissue. The results support a role for the choroid plexus in the interactions between PL and the CNS.