The electrophysiological properties of cardiac muscle in KK/Ta mouse (hereafter referred to as KK mouse), an animal model of human non-insulin-dependent diabetes mellitus, were investigated, and the findings compared with those obtained from a non-diabetic control mouse (C57BL/6J mouse; referred to as B6 mouse). The ages of the B6 mice were 23.9 ± 5.4 weeks (n=24) and those of the KK mice used were 25.7 ± 10.8 weeks (n=34). The KK mice had mild obesity, hyperglycemia and hyperinsulinemia. Ventricular muscles from both mice were examined by light microscopy. Partial myocardial fibrosis and filament disorder in the ventricular muscles were found only in the KK mice. The resting membrane potential of the ventricular muscle was less negative in the KK mice than in the control mice. The maximum rate of rise in the upstroke of the action potential was significantly decreased in the KK mice compared with that of the control mice. These suggest a decrease in a time-independent K⁺ current (I_K1) in the KK mice. The duration of the action potential (APD) at all levels of repolarization was significantly longer in the KK mice than in the B6 mice. A blocker of transient outward current (I_to), 4-aminopyridine, significantly prolonged the APD of the B6 mice, but failed to prolong it in the KK mice, suggesting that I_to in the diabetic mice is very small. A Ca²⁺ channel blocker, CoCl₂, dramatically lengthened all levels of APD in both groups, suggesting that there is no difference between B6 mice and KK mice in L-type Ca²⁺ current via Ca²⁺ channels. These suggest the malfunction or deficiency of ionic channels which carry, at least, I_to and I_K1 in diabetic mice.