A study was made of 14 thoracotomized dogs under i.v. sodium thiopental anesthesia; the effects of 1.5 mg/kg intravenous ATP on sinus node automatism and atrio-ventricular conduction were investigated. In 7 dogs (group A) ATP was administered under control conditions and following successive intravenous administrations of atropine (1 mg/kg), aminophylline (5 mg/kg) and propranolol (0.6 mg/kg). The remaining 7 dogs (group B) received ATP following atropine (1 mg/kg), isoproterenol (0.4 microgram/kg/min.), and aminophylline (5 mg/kg). An analysis was made of the percentage variations in cardiac cycle length during spontaneous rhythm and of the AH interval during atrial pacing at a fixed rate. In group A the negative dromotropic and chronotropic effects of ATP under control conditions decreased in 5 cases following atropine, although the average decrease was not statistically significant. On adding aminophylline, a statistically significant decrease was observed in the effects of ATP, and following propranolol the drop in negative chronotropic effect of ATP provoked by aminophylline was maintained. In group B, and following prior atropinization, the negative chronotropic and dromotropic effects of ATP were maintained in the presence of isoproterenol. As in group A, aminophylline significantly reduced the effects of ATP. To conclude: in the thoracotomized dog under sodium thiopental anesthesia, 1) atropine does not prevent the negative chronotropic and dromotropic actions of ATP, although the effect of the latter is decreased in a large percentage of cases; 2) sympathetic beta stimulation following prior atropinization does not prevent ATP action; 3) aminophylline in the atropinized dog noticeably reduces the effects of ATP, and 4) this action of aminophylline is effective in the presence of sympathetic beta stimulation.