Recent evidence suggests that the peptide hormone prolactin (PRL) modulates energy balance through a number of mechanisms, including acting in the brain to increase food intake. In the current studies, we first demonstrated that chronic infusions of PRL into the lateral ventricles increased food intake in cycling rats without disrupting estrous cyclicity. In subsequent experiments the hypothesis that at least part of PRL's ability to increase food intake resulted from PRL-induced leptin resistance was tested. Female rats given chronic infusions of PRL (5 microg/h) into the cerebral ventricles for 10 d did not show a reduction in food intake or body weight after a central injection of 4 microg murine leptin, whereas the expected reduction in both of these parameters was seen in vehicle-infused rats. Leptin injections were without effect on these parameters, whether they were administered to free feeding PRL-infused rats or after 24-h food deprivation. This lack of a behavioral response to leptin was accompanied by an attenuation in Fos induction and phosphorylation of signal transducer and activator of transcription 3 after leptin administration in PRL-infused rats in both the ventromedial hypothalamus and paraventricular hypothalamic nucleus.