The arrhythmogenic and hemodynamic effects of 0.04% aconitine solution applied locally to the left atria of cats and the effects of amiodarone (10 mg/kg) and atropine (0.2 mg/kg) on the responses to aconitine were investigated. Cats were anesthetized with sodium pentobarbital. The left atrial and limb lead electrocardiograms and the arterial pressure were recorded. Cardiac output was determined with the thermo dilution technique. Aconitine nitrate induced dysrhythmias lasting for a mean of 111 min. Atrial fibrillation occurred in 47% of the animals. A long-lasting (at least 3 hr) cardiodepressant action was seen in response to aconitine. Pretreatment with atropine largely prevented the hemodynamic effects of aconitine and prevented fibrillation. However, mean maximum atrial rates recorded during successive 5 min intervals were similar in control and atropine-treated animals. Amiodarone suppressed dysrhythmias in both atropine-treated and nontreated animals at a dose which exerted only minimal cardiodepression. The method described for evaluating potential activity against supraventricular dysrhythmias in the cat is relatively simple, reproducible, and suitable for statistical analyses. Aconitine appears to exert a long-lasting cholinergic action which may be involved in the genesis of aconitine-induced atrial fibrillation.