In this study, the role of the RhoA/Rho-kinase (RhoA/ROCK)-signaling pathway in cardiovascular dysfunction associated with hyperthyroidism was examined with the use of fasudil, a Rho-kinase inhibitor. Male Spraque-Dawley rats were treated with l-thyroxine (T4) alone, T4 + low-dose fasudil (2 mg/kg/day) or T4 + high-dose fasudil (10 mg/kg/day) and compared with control animals. Rats in the T4 group showed an increase in the ratio of heart weight to body weight, which was ameliorated by fasudil at both low and high doses. Morphometric and hemodynamic parameters were also evaluated and confirmed that fasudil attenuated the cardiac hypertrophy induced by T4. The extent of phosphorylation of the myosin phosphatase targeting subunit was quantified by Western blotting to evaluate the activity of Rho-kinase in the heart tissue. Both Western blotting and reverse transcriptase–polymerase chain reaction analyses revealed enhancement of Rho-kinase and activator protein 1 activity and reduction of c-FLIPL expression in the T4 group, and this response was inhibited by f asudil in a dose-dependent manner. Furthermore, f asudil inhibited apoptosis induced by T4 as evidenced by the detection of terminal deoxynucleotidyl transferase dUTP nick end labeling–positive cells and the expressions of bax and bcl-2. These results suggested that the RhoA/ROCK pathway is involved in the cardiac hypertrophy induced by experimental hyperthyroidism. The antagonism of this pathway may thus be useful as an alternative target in the treatment of hyperthyroid heart disease.