AIM　To study the characteristic of th e rapidly activating component (Ikr) and the slowly activating compone nt (Iks) of the delayed rectifier K+ current in guinea pig hypertrop hi ed ventricular myocytes induced by thyroxine. METHOD　The whole c ell patch clamp techniques were used. RESULTS　In hypertrophied ve ntricular myocytes, the magnitude of IKr and IKs were great ly augmented with more positive depolarizations, and the degree of increase of IKr tail and IKs were greater than that of IKr an d IKs tail, respectively. By measuring the amplitude of tail currents which reflected the degree of activation, the activation curve of IKs was shifted toward more negative potential, but that of IKr was marked ly unaffected in hypertrophied myocytes. CONCLUSION　Hypertrophied ventricular myocytes induced by thyroxine obviously increased IKr and IKs.