Class Ia phosphoinositide (PI) 3-kinases are heterodimers composed of a regulatory and a catalytic subunit and are essential for the metabolic actions of insulin. In addition to p85α and p85β, insulin-sensitive tissues such as fat, muscle, and liver express the splice variants of the pik3r1 gene, p50α and p55α. Το define the role of these variants, we have created mice with a deletion of p50α and p55α by using homologous recombination. These mice are viable, grow normally, and maintain normal blood glucose levels but have lower fasting insulin levels. Results of an insulin tolerance test indicate that p50α/p55α knockout mice have enhanced insulin sensitivity in vivo, and there is an increase in insulin-stimulated glucose transport in isolated extensor digitorum longus muscle tissues and adipocytes. In muscle, loss of p50α/p55α results in reduced levels of insulin-stimulated insulin receptor substrate 1 (IRS-1) and phosphotyrosine-associated PI 3-kinase but enhanced levels of IRS-2-associated PI 3-kinase and Akt activation, whereas in adipocytes levels of both insulin-stimulated PI 3-kinase and Akt are unchanged. Despite this, adipocytes of the knockout mice are smaller and have increased glucose uptake with altered glucose metabolic pathways. When treated with gold thioglucose, p50α/p55α knockout mice become hyperphagic like their wild-type littermates. However, they accumulate less fat and become mildly less hyperglycemic and markedly less hyperinsulinemic. Taken together, these data indicate that p50α and p55α play an important role in insulin signaling and action, especially in lipid and glucose metabolism.