The long form of the leptin receptor (Ob-Rb) has a cytoplasmic domain which activates the JAK-STAT signal transduction pathway. It is related to appetite and energy expenditure and is expressed in various parts of the brain in adults. In embryos, however, the detailed distribution of Ob-Rb expression sites and the function of the leptin-Ob-Rb system remain unclear, although leptin is detected in human cord plasma and leptin mRNA is detected in mouse embryos. In this study, we investigated the Ob-Rb mRNA expression pattern in the brains of mouse embryos and newborn mice by RT-PCR and in situ hybridization. At embryonic day 10.5 (E10.5), Ob-Rb mRNA was already detected in the brain by RT-PCR. By in situ hybridization, Ob-Rb mRNA was observed in the ventricular zone of the rhombencephalon at E11.5. At E12.5, it was also expressed in the ventricular zone of the telencephalon, mesencephalon and cerebellar primordium. From E14. 5 it was expressed in the cortical plate of the telencephalon and the ventricular zone of the thalamus. At E16.5, it was expressed in the premamillary hypothalamic nucleus, superficial gray matter of the superior colliculus, external germinal and Purkinje cell layers of the cerebellum, and facial nucleus. At E18.5, it was expressed in the arcuate nucleus and ventromedial hypothalamic nucleus. These results suggest that the leptin-Ob-Rb system is related to brain development.