The metabolism of actisomide, a novel antiarrhythmic agent, was studied in the dog, monkey and man and was found to be more extensive in the monkey than in the dog or man. The major metabolites identified were a piperidinyl hydroxylated metabolite, the mono-N-dealkylated, cyclized and piperidine hydroxylated metabolite, and the cyclized and mono-N-dealkylated metabolite. Excretion of the parent drug was higher in urine than in feces in the dog, but in the monkey and man, urinary and fecal excretion of actisomide was similar. In all species the metabolites were primarily excreted in feces.