Thiazolidinedione derivatives are insulin-sensitizing agents with proven antidiabetic activities in vivo. To explore the mechanism of action of this class of compounds, the effects of pioglitazone, CP-86,325, and AD-5075 on elements of the insulin signal transduction pathways were studied in Chinese hamster ovary cells overexpressing human insulin receptor (CHO.T) and L6 myotubes. In CHO.T cells, the binding of insulin to its receptor and the insulin-stimulated tyrosine kinase activity of the receptor were not altered by pioglitazone or CP-86,325. In contrast, treatment of CHO.T cells with the compounds resulted in significant increases in insulin-stimulated phosphatidylinositol (PI) 3-kinase activity. This insulin-enhancing effect was also observed in L6 myotubes treated with CP-86,325. The augmentations in kinase activity observed in CHO.T cells correlated with increases in the amount of PI 3-kinase (p85 subunit) in anti-phosphotyrosine immunoprecipitates of cell lysates. No gross changes in the tyrosine phosphorylation state of the insulin receptor substrate-1 were detected in insulin-stimulated CHO.T cells following treatment with the compounds. Furthermore, the compounds did not enhance insulin stimulation of mitogen-activated protein kinase or DNA synthesis in CHO.T cells. Thus, thiazolidinedione-derived antidiabetic agents may act as insulin sensitizers by augmenting insulin stimulation of PI 3-kinase activity in a rather specific manner.