The activity of adenosine and inosine on coronary blood flow and arterial blood pressure was investigated in anaesthetized and thoracotomized dogs. The following results were obtained. 1. Individual adenosine administration caused an increase in coronary blood flow. However, this adenosine activity was significantly strengthened when the same dose of adenosine was applied simultaneously with inosine in doses of 3.2, 5.6 and 10.0 mg/kg body weight i.v. The duration of the effect of adenosine on the coronary blood flow was also potentiated by inosine. 2. The hypotensive effect resulting from 0.2 mg adenosine/kg i.v. was significantly strengthened by simultaneous inosine application (3.2, 5.6 and 10.0 mg/kg i.v.). The activities on the blood pressure of increments in the individual adenosine dosages (0.2, 0.4 and 0.6 mg/kg i.v.) were significantly potentiated by 10 mg inosine/kg body weight i.v. 3. The heart rate was not modified by 0.2 mg adenosine/kg i.v. However, the same adenosine dosage plus inosine (3.2, 5.6 and 10.0 mg/kg i.v.) applied simultaneously led to bradycardia. Increasing adenosine doses (0.2, 0.4 and 0.6 mg/kg i.v.) applied simultaneously with inosine (10.0 mg/kg body weight i.v.) led to a dose dependent retardation of the heart rate. 4. The possible reasons for the potentiation of the effect of adenosine by inosine were discussed.