The genetically obese Koletsky rat (SHROB, fak) has a novel point mutation of the leptin receptor at amino acid +763, resulting in a premature stop codon in the leptin receptor extracellular domain. This implies that all leptin receptor isoforms should be absent in this model. We examined the phenotypic consequences of this mutation on leptin and leptin receptor mRNA in hypothalamus and peripheral tissues from SHROB and their lean SHR littermates. Despite the mutation, mRNA for both the long (ObRa) and the short (ObRb) form were expressed at comparable levels in SHROB and SHR in brain and throughout peripheral tissues. Adipose tissue mRNA for leptin was two to three times greater in SHROB compared to SHR (P < 0.01), while circulating leptin concentration was 170 times greater than SHR littermates (P < 0.01), suggesting extreme leptin resistance in SHROB. Leptin was also detected in the cerebrospinal fluid (CSF) of SHR and SHROB (13.8 and 27.2 pmol/L, respectively); however, the CSF/plasma ratio for leptin was 32-fold greater in SHR than in SHROB. To assess the putative action of leptin and leptin receptors on insulin-mediated glucose transport, muscles from SHR and SHROB were incubated in vitro with recombinant human leptin. Leptin directly suppressed insulin-mediated glucose transport by 50% in skeletal muscle from SHR but not in obese SHROB rats lacking all forms of the leptin receptor. These results suggest that the natural leptin receptor knockout in the SHROB represents a unique rat model to define the functional role(s) of leptin in central and peripheral energy metabolism.