Abstract: The pharmacological effects of cardiac glycosides on coronary vessels are controversial. ⁸⁶RbCI was utilized to evaluate the influences of convallatoxin and ouabain on myocardial microvascular bed.Experiments were performed on 430 mice each weighing 18-24 g. When convallatoxin was given iv 0.05 and 0.1 mg/kg, the mean ratio of myocardial uptake of ⁸⁶RbCl was significantly increased within 1 minute, reaching a peak at 15-30 minutes. The duration of elevation was more than 1 hour (table 1). When convallatoxin was iv 0.5 and 1.0 mg/kg, arrhythmia appeared in most mice. The elevation of myocardial uptake of ⁸⁶RbCI was decreased and reached the peak at 15 minutes. Ouabain produced a similar effect.Thus, small doses (0.05 and 0.1 mg/kg) of convallatoxin and ouabain dilated the myocardial microvascular bed and increased the myocardial nutritional blood perfusion, while larger doses (0.5 and 0.1 mg/kg) of convallatoxin and ouabain yielded an opposite effect.