Afferent activities were recorded in vagosympathetic nerves from mechanoreceptors of the isolated heart of bullfrog. The rate of discharges was increased when Ca2+ or Mg2+ (1.1 approximately 4.4 mM) was added to normal Ringer's solution containing 1.1 mM Ca2+. In terms of increase in the rate of discharges following the addition of Ca2+, an increase in the tone of cardiac muscle at diastole was observed. The rate of discharges was decreased and thereafter abolished within 5 to 10 min by lowering Ringer's solution Ca2+ concentration from 1.1 to 0.37 mM. A high firing frequency was observed with the addition of Mg2+ (0.73 mM) in the low Ca2+ Ringer's solution containing 0.37 mM Ca2+ and the rate remained unchanged for at least 15 min. On the other hand, the discharges were reduced, concentration dependently, when Mn2+ (0.22 approximately 1.1 mM) was added to normal Ringer's solution. When Mn2+ (0.22 or 0.55 mM) was applied, the rate of discharges decreased gradually and the reduction of discharges decreased 10 to 20 min after application. It appears that increase in the rate of discharges by application of Ca2+ is mainly the result of an increase in the tone of cardiac muscle induced by Ca2+. The presence of Ca2+ in external medium seems to be necessary to maintain the generation afferent discharges and Mg2+ can act as a substitute for Ca2+ in the generation of afferent discharges. The inhibitory effect of Mn2+ seems to be the result of depression of mechanoreceptors but the receptor becomes insensitive to Mn2+ within 10 to 20 min during exposure to low concentrations of Mn2+.