In isolated guinea pig right papillary muscle, the sodium-carrying system (fast inward current) was inactivated by increasing the external potassium concentration from 5.4 to 16 mM (decrease of an equivalent amount of NaCl). The electrical response was recorded with application of relatively strong cathodal current pulses of 10 or 400 msec duration to the muscle across a partition. The results obtained were; (1) the amplitude of the electrical response increased depending on the strength of the stimulation; (2) the mean rate of rise and the overshoot of this response increased markedly with a high concentration of external calcium; (3) the mean rate of rise (Ca2+ = 7.2 mM; 2.01 V/sec) was considerably smaller than that of the action potential; and (4) this electrical response was not affected by tetrodotoxin (4 x 10(-6) M), but was depressed by Mn2+ (3 x 10(-3) M). These results suggested that the electrical response (slow response) was due to activation of the slow component of inward current, mainly carried by Ca2+. The effect of dobutamine (DOB) on the slow response was studied in vitro, then compared with effects of dopamine (DA), norepinephrine (NE) and isoproterenol (ISO). These catecholamines (CA) increased the amplitude of the slow response and the sensitivity to electrical stimulus strength. Augmentative effects of these drugs were in the following order of ISO greater than NE greater than DOB greater than DA. The finding that DA and ISO increase the sensitivity to electrical stimulus strength more than the amplitude of slow response will be researched in further investigations.