The G protein α-subunit Gsα is required for hormone-stimulated cAMP generation. The Gsα gene Gnas is a complex gene with multiple imprinted gene products. Mice with heterozygous disruption of the Gnas paternal allele (+/p-) are partially Gsα deficient and totally deficient in XLαs, a neuroendocrine-specific G sα isoform that is expressed only from the paternal Gnas allele. We previously showed that these mice are hypermetabolic and lean and have increased insulin sensitivity. We now performed hyperinsulinemic-euglycemic clamp studies, which confirmed the markedly increased whole body insulin sensitivity in +/p- mice, +/p- mice had 1.4-, 7- and 3.8-fold increases in insulin-stimulated glucose uptake in muscle and white and brown adipose tissue, respectively, and markedly suppressed endogenous glucose production from the liver. This was associated with increased phosphorylation of insulin receptor and a downstream effector (Akt kinase) in both liver and muscle in response to insulin. Triglycerides cleared more rapidly in +/p- mice after a bolus administered by gavage. This was associated with decreased liver and muscle triglyceride content and increased muscle acyl-CoA oxidase mRNA expression. Resistin and adiponectin were overespressed in white adipose tissue of +/p- mice, although there was no difference in serum adiponectin levels. The lean phenotype and increased insulin sensitivity observed in +/p- mice is likely a consequence of increased lipid oxidation in muscle and possibly other tissues. Further studies will clarify whether XLaαs deficiency is responsible for these effects and if so, the mechanism by which XLαs deficiency leads to this metabolic phenotype.