1. Conscious or barbiturate-anaesthetized rabbits were slowly infused intravenously with solutions of amitriptyline, imipramine, maprotiline or mianserin, usually until death occurred. 2. Amitriptyline produced death at the lowest dose, imipramine and maprotiline were intermediate while much higher doses of mianserin were required. 3. Convulsions were induced by the antidepressants in all conscious rabbits and the order of potency of the drugs in producing this effect was amitriptyline greater than or equal to imipramine greater than maprotiline greater than mianserin. 4. All four drugs produced a reduction in heart rate and blood pressure in the anaesthetized rabbits and the order of potency in this respect was amitriptyline greater than imipramine greater than maprotiline greater than mianserin. 5. All four drugs produced significant changes in the ECG compared with control rabbits. The P-R interval was lengthened (potency order amitriptyline greater than imipramine greater than or equal to maprotiline greater than mianserin) and the QRS complex was widened (potency order amitriptyline greater than imipramine greater than or equal to maprotiline greater than mianserin). 7. It is concluded that all four drugs show the toxic effects classically associated with tricyclic antidepressants but the relative toxicity amongst these agents varies considerably and is in the order amitriptyline greater than imipramine greater than maprotiline greater than mianserin.