The serine-threonine kinase Akt, also known as protein kinase B (PKB), is an important effector for phosphatidylinositol 3-kinase signaling initiated by numerous growth factors and hormones. Akt2/PKBbeta, one of three known mammalian isoforms of Akt/PKB, has been demonstrated recently to be required for at least some of the metabolic actions of insulin (Cho, H., Mu, J., Kim, J. K., Thorvaldsen, J. L., Chu, Q., Crenshaw, E. B., Kaestner, K. H., Bartolomei, M. S., Shulman, G. I., and Birnbaum, M. J. (2001) Science 292, 1728-1731). Here we show that mice deficient in another closely related isoform of the kinase, Akt1/PKBalpha, display a conspicuous impairment in organismal growth. Akt1(-/-) mice demonstrated defects in both fetal and postnatal growth, and these persisted into adulthood. However, in striking contrast to Akt2/PKBbeta null mice, Akt1/PKBalpha-deficient mice are normal with regard to glucose tolerance and insulin-stimulated disposal of blood glucose. Thus, the characterization of the Akt1 knockout mice and its comparison to the previously reported Akt2 deficiency phenotype reveals the non-redundant functions of Akt1 and Akt2 genes with respect to organismal growth and insulin-regulated glucose metabolism.