Leptin concentrations increase during pregnancy, but this does not prevent the pregnancy-induced increase in food intake, suggesting a state of leptin resistance. This study investigated the response to intracerebroventricular leptin administration in pregnant rats. After fasting, nonpregnant, d-7 and d-14 pregnant rats received leptin (4 microg) or vehicle, then food intake was measured. Serial blood samples were collected in another group of rats to determine plasma leptin concentrations. Further groups of d-14 pregnant and nonpregnant rats were killed after leptin or vehicle treatment, and brains were collected. Hypothalamic nuclei were microdissected, and levels of signal transducer and activator of transcription (STAT)3 phosphorylation were measured using Western blot analysis. Fasting decreased leptin concentrations in both pregnant and nonpregnant rats. Leptin treatment significantly reduced food intake in nonpregnant and d-7 pregnant rats but not in d-14 pregnant rats. In addition, there was no postfasting hyperphagic response in the pregnant rats. In the pregnant rats, leptin-induced STAT3 phosphorylation was suppressed in the arcuate nucleus and, to a lesser extent, in the ventromedial hypothalamus (VMH), compared with nonpregnant rats. Unstimulated STAT3 levels were also decreased in the VMH during pregnancy. Leptin-induced phosphorylation of STAT3 in the dorsomedial and lateral hypothalamus was not different between pregnant and nonpregnant rats. These data indicate that pregnant rats become resistant to the satiety action of leptin. Furthermore, leptin-induced activation of the STAT3 is impaired during pregnancy, specifically in the arcuate nucleus and VMH. These data support the hypothesis that pregnancy is a state of hypothalamic leptin resistance.