Recently, attention of many investigators has been directed to the physiological significance of the protein sulfhydryl (SH) group in various tissues. In the heart, too, the solution of this problem was pursued by several workers (1,2,3,4,5). The effect of cadmium chloride on the electrical activity of the frog's heart was reported by Logunova and Kipershlak (4), who postulated that the electrocardiographic changes with cadmium chloride was due to a binding of the protein sulfhydryl (SH) groups. Afterwards, Kleinfeld et al. (5) revealed the similar disturbance with cadmium chloride in the single myocard fiber, using the microelectrode technique.The following experiments were undertaken, a) to compare the effect of cadmium chloride on the toad's heart with that of mercuric derivatives, such as p-chloromercurobenzoic acid and phenylmercuric acetate which are known to be more specific sulfhydryl reagents, and b) to investigate in details the dissociation of the mechanogram and electrogram of the heart with sulfhydryl reagents. METHODSThe isolated heart of the Japanese toad in winter stage was perfused through Yagi's recurrent perfusion cannula with Ringer solution which contained a test substance of a desired concentration.The cannula was always filled with the fixed volume (5ml.) of perfusates which were adjusted to pH6.8.To transform mechanograms into electrical events, the shadow of a contracting heart, that was exposed to parallel light beams entering through a narrow slit, was projected into a phototube, connected with a three stages amplifier with a time constant of3seconds.Thus, the electrocardiogram and at the same time, the mechanogram were recorded with a ink writing oscillograph, of which natural frequency was70cps.As leading electrodes, the cotton thread/AgC1/Ag type was used.To lead the bipolar electrocardiogram, cotton thread electrodes moistened with Ringer solution were placed on the wall of the auricle and at the apex of the ventricle. For monophasic leading, the indifferent electrode was soaked with isotonic KC1