Abstract: AIM：To study the effect of amiloride on L-thyroxine induced hypertrophic myocytes in rats. METHODS: Myocardial hypertrophy was induced by L-thyroxine (1 mg.kg^-1.d^-1, ip) in rats. The changes of morphology of rat myocytes under microscope was observed. For recording the action potential of rat papillary muscle, microelectrode technique was used. RESULTS: After administration of thyroxine in rats, the morphology and duration of action potential of the papillary muscle (especially to APD_20,50,90) changed. At the dose of 0.5　mg.kg^-1.d^-1, amiloride(po) was shown to decrease the degree of myocardial hypertrophy and prevent APD₂₀ of hypertrophic papillary from prolongation. CONCLUSION: At the dose of 0.5 mg.kg^-1.d^-1, amiloride(po) was shown to be effective against myocardial hypertrophy with methods of morphology and electrophysiology.