Effects of local anesthetics, tetrodotoxin (TTX), aconitine and verapamil on the rate of afferent discharges from the mechanoreceptors of isolated hearts of the bullfrog were studied. When procaine (1 X 10(-4) M), tetracaine (1 X 10(-6) M) and dibucaine (1 X 10(-5) M) decreased the contractile force and beating rate of the heart, high frequency discharges that were synchronized with the contraction of the heart appeared. When concentrations of these local anesthetics were increased, the afferent discharges were abolished. When TTX (1 X 10(-8) M - 1 X 10(-7) M) caused a cardiac arrest, high frequency discharges, consisting of maintained discharges and the discharges synchronized with small contractions of the ventricle, appeared. Occasionally, in the presence of TTX (1 X 10(-7) M) the cardiac arrest occurred, and then very irregular contractions appeared. At that time, high frequency discharges synchronized with contractions appeared. On the other hand, aconitine (1 X 10(-7) M - 1 X 10(-6) M) initially increased the rate of afferent discharges from mechanoreceptors in the atrium and ventricle and then abolished the discharges without significantly affecting beating rhythms and contractile force. When verapamil (1 X 10(-6) M - 1 X 10(-5) M) was applied, the beating rate and contractile force were significantly decreased, but high frequency discharges synchronized with contractions were observed. These results indicate that local anesthetics, TTX and verapamil at those concentrations which depressed the cardiac functions did not inhibit the heart mechanoreceptor excitability and the concentration of aconitine that markedly affected the cardiac functions stimulated the heart mechanoreceptor.