Protein kinase B (PKB) is known to mediate a number of biological responses to insulin and growth factors, its role in glucose uptake being one of the most extensively studied. In this work, we have employed a recently described allosteric inhibitor of PKB, Akti, to clarify the role of PKB in lipid metabolism in adipocytes-a subject that has received less attention. Pretreatment of primary rat and 3T3L1 adipocytes with Akti resulted in dose-dependent inhibition of PKB phosphorylation and activation in response to insulin, without affecting upstream insulin signaling [insulin receptor (IR), insulin receptor substrate (IRS)] or the insulin-induced phosphoinositide 3-kinase (PI3K)-dependent activation of the ERK/p90 ribosomal kinase (RSK) pathway. PKB activity was required for the insulin-induced activation of phosphodiesterase 3B (PDE3B) and for the antilipolytic action of insulin. Moreover, inhibition of PKB activity resulted in a reduction in de novo lipid synthesis and in the ability of insulin to stimulate this process. The regulation of the rate-limiting lipogenic enzyme acetyl-CoA carboxylase (ACC) by insulin through dephosphorylation of S79, which is a target for AMP-activated protein kinase (AMPK), was dependent on the presence of active PKB. Finally, AMPK was shown to be phosphorylated by PKB on S485 in response to insulin, and this was associated with a reduction in AMPK activity. In summary, we propose that PKB is required for the positive effects of insulin on lipid storage and that regulation of PDE3B and AMPK by PKB is important for these effects.