To investigate the association between hyperinsulinemia and cardiac hypertrophy, we treated rats with insulin for 7 weeks and assessed effects on myocardial growth, vascularization, and fibrosis in relation to the expression of angiotensin II receptors (AT-R). We also characterized insulin signaling pathways believed to promote myocyte growth and interact with proliferative responses mediated by G-protein-coupled receptors, assessed myocardial IRS-1, p110 catalytic and p85 regulatory subunits of PI3K, Akt, MEK, ERK1/2, and S6K1. Left ventricular (LV) geometry and performance were evaluated echocardiographically. Insulin decreased AT1a-R mRNA expression but increased protein levels and increased AT2-R mRNA and protein levels and phosphorylation of IRS-1 (Ser 374/Tyr 989), MEK1/2 (Ser 218/Ser 222), ERK1/2 (Thr 202/Tyr 204), S6K1 (Thr 421/Ser 424/Thr 389), Akt (Thr 308/Thr 308) and PI3K p110 but not of p85 (Tyr 508). Insulin increased LV mass, relative wall thickness, and reduced stroke volume and cardiac output. Histochemical examination demonstrated myocyte hypertrophy and increases in interstitial fibrosis. Metoprolol plus insulin prevented the increase in relative wall thickness, decreased fibrosis, increased LV mass, and improved function seen with insulin alone. Thus, our data demonstrate that chronic hyperinsulinemia decreases AT1a/AT2 ratio and increases MEK/ERK1/2 and S6K1 pathway activity related to hypertrophy. These changes might be crucial for increased cardiovascular growth and fibrosis and signs of impaired LV function.