The responsiveness of the hypothalamus to the inhibitory effects of leptin on food intake and body weight is influenced by multiple factors, including deficiency of either leptin or leptin receptors (Ob-R). To investigate whether altered expression of Ob-R in the hypothalamus could potentially contribute to altered leptin sensitivity, we performed in situ hybridization with riboprobes that detected either mRNAs encoding both the long (Ob-Rb) and short (Ob-Ra) splice variants or mRNA encoding only Ob-Rb. In the arcuate nucleus, mRNA encoding Ob-Rb, the predominant signaling form of the receptor, was 2.3 times greater in obese db/db and ob/ob mice than in lean +/ob controls (P < 0.01). In ob/ob mice, systemic administration of leptin reduced Ob-Rb mRNA content of the arcuate nucleus by 30% compared with saline-treated, pair-fed controls (P < 0.05). A 48-h fast increased Ob-Rb mRNA levels in the arcuate nucleus of normal and neuropeptide Y (NPY)-knockout mice (P < 0.01), although the effect was greater in the NPY-knockout mice (400 vs. 247%, P < 0.05). In addition, Ob-Rb mRNA hybridization was elevated by 40% in the arcuate nucleus (P < 0.05) and by 75% in the ventromedial nucleus (P < 0.05) of rats fasted 48 h. The results suggest that expression of Ob-Rb mRNA in the hypothalamus is sensitive to genetic and physiological interventions that alter circulating leptin levels, and that overexpression of Ob-Rb in the hypothalamus may contribute to increased leptin sensitivity.