HMG-CoA reductase inhibitors, so called statins, decrease cardiac events. Previous studies have shown that HMG-CoA reductase inhibitors inhibit cardiomyocyte hypertrophy in vitro and in vivo by blocking Rho isoprenylation. We have shown that the G1 cell cycle regulatory proteins cyclin D1 and Cdk4 play important roles in cardiomyocyte hypertrophy. However, the relation between Rho and cyclin D1 in cardiomyocyte is unknown. To investigate whether HMG-CoA reductase inhibitors prevent cardiac hypertrophy through attenuation of Rho and cyclin D1, we studied the effect of fluvastatin on angiotensin II-induced cardiomyocyte hypertrophy in vitro and in vivo. Angiotensin II increased the cell surface area and [(3)H]leucine uptake of cultured neonatal rat cardiomyocytes and these changes were suppressed by fluvastatin treatment. Angiotensin II also induced activation of Rho kinase and increased cyclin D1, both of which were also significantly suppressed by fluvastatin. Specific Rho kinase inhibitor, Y-27632 inhibited angiotensin II-induced cardiomyocyte hypertrophy and increased cyclin D1. Overexpression of cyclin D1 by adenoviral gene transfer induced cardiomyocyte hypertrophy, as evidenced by increased cell size and increased protein synthesis; this hypertrophy was not diminished by concomitant treatment with fluvastatin. Infusion of angiotensin II to Wistar rats for 2 weeks induced hypertrophic changes in cardiomyocytes, and this hypertrophy was prevented by oral fluvastatin treatment. These results show that an HMG-CoA reductase inhibitor, fluvastatin, prevents angiotensin II-induced cardiomyocyte hypertrophy in part through inhibition of cyclin D1, which is linked to Rho kinase. This novel mechanism discovered for fluvastatin could be revealed how HMG-CoA reductase inhibitors are preventing cardiac hypertrophy.