Higenamine is known to possess stimulatory activity on beta-receptor of the heart. Chronotropic actions of higenamic were studied in spontaneously beating right atrial muscle isolated from rabbits. The frequency of spontaneous beating and the relative threshold voltage of the right atrium were examined. The relative threshold voltage was defined as the minimal voltage of the given impulse above which the right atrium could be paced at the frequency of 20% of higenamine was also observed. Higenamine caused the postive chronotropic effect. This response became prominent as the ca2+ concentration in the bathing solution lowered. When tetrodotoxin was added to the bathing solution, the effect of higenemine altered and became similar to that of epinephrine. Higenamine reduced the relative threshold voltage of the right atrium in the bathing solution with [ca2+] of 0.5mM. Such effect was abolished by tetrodotoxin. The effects of verapamill on the spontaneous rate and the relativel threshold voltage were inhibited by higenemine. The above results suggest that, aithough the main action og higenamine is on the Ca channel, higenamine also have a minor effect of augmenting the Na channel.