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SUMMARYTwo patients who have undergone heart transplantation 7 and 15 months previously were studied to determine basal heart rate, the effects of exercise, and acute changes in venous return and blood pressure as well as of vagal stimulation. In one case the effects of body temperature, isoprenaline (isoproterenol), beta blockade, and atropinization on heart rate were studied. Mean resting heart rates of 110 and 90, respectively, were observed in the two cases without any significant change with time since the surgical implantation. Sinus arrhythmia was not observed and heart rate was found to increase with exer'cise following isoprenaline infusion and following increase in-body temperature. A'decrease was observed after beta blockade, but no significant direct or baroreceptor reflex effects could be produced by changes in venous return, systemic blood pressure,, vagal stimulation, and atropinization. Both patients are thus effectively denervated, and there is no evidence of reinnervation to date.