The metabolism of 74As-arsenate (As V, 0.4 mg As/kg body weight, IV) in marmoset monkeys (two males and two females) was studied. Unlike all other animal species studied so far, the marmoset was found to be unable to metabolize the arsenate to dimethylarsinic acid. Most of the absorbed arsenate was reduced to arsenite (As III) in vivo. Only 20% was excreted in the urine as unchanged As V. A further 20% of the dose was excreted as As III. The rest of the As III produced was bound to the tissues, giving a distribution picture very similar to that reported earlier for marmoset monkeys given arsenite. The tissues with longest retention of arsenic were the liver, upper gastrointestinal tract (oral cavity and esophagus), skin, kidneys and gall bladder. The pronounced accumulation in the liver resulted from specific binding of arsenic to the rough microsomal membranes, unique to this animal species.