The experiments were performed to investigate the effects of cyanide and several antidotes (sodium nitrite, DMAP, sodium thiosulfate and phentolamine) on isolated perfused rat heart to further elucidate the mechanism of action of cyanide and its antidotes. Sodium cyanide (40, 50 and 60 mg/ml) produced maximum effects on amplitude, heart rate and coronary flow, after 3 min, regardless of the event recorded. Some dose-dependence was noted, especially on the heart rate, somewhat less on amplitude and almost none on the coronary flow up to 15 min, but notable afterwards. Some spontaneous recovery occurred in each event, the most significant being that of the heart rate. Antidotes per se did not differ significantly in their action on isolated rat heart. All values, with some variations, were maintained within 10-20% of control. Antidotes, injected 1 min after sodium cyanide produced the most evident effects on amplitude and heart rate -- all antidotes practically and almost instantly neutralized the depressant effect of cyanide. Nitrite and particularly DMAP, were the most effective on coronary flow while other antidotes, although effective up to 3 min, did not much differ from cyanide alone throughout the rest of observation.