Conscious rabbits which had been permanently catheterized into their aortas and posterior caval veins, were injected daily with 10 mg/kg of protriptyline subcutaneously, divided in 3 doses. The blockade of the membrane pump in sympathetic nerve terminals by protriptyline was checked by pressor tests with noradrenaline (NA) and tyramine. In the presence of the membrane pump blockade 2.5 mg/kg of amitriptyline, nortriptyline, or protriptyline, or 3.0 mg/kg of doxepin was injected i.v. The antidepressants lowered blood pressure transiently and increased the heart rate, doxepin and amitriptyline being more effective than nortriptyline and protriptyline. Amitriptyline and doxepin provoked more severe cardiac arrhythmias on ECG than nortriptyline, and protriptyline caused no arrhythmias. Intravenous infusion of NA (11 mug/min) raised the blood pressure and lowered the heart rate. Injection of antidepressants during NA infusion resulted in more pronounced depressor and tachycardic effects than occurred without NA infusion. Major ECG changes were only slightly more apparent than without NA infusion. The rank order of toxicity of the antidepressants was the same. It is concluded that the NA potentiation by tricyclic antidepressants is not the main reason for their cardiotoxic effects.