The distribution, excretion, and possible metabolism of (14)C- and/or (74)As-cacodylic acid, and organoarsenical herbicide, was studied in rats following a single intravenous injection, intratracheal instillation or oral gavage. Male Sherman rats were dosed at levels ranging from 200 mg/kg to 120 micrograms/kg. The extent and rate of lung absorption was greater than gastrointestinal absorption. Concentrations in the liver and whole blood were higher after peroral dosing than intravenous administration. Levels observed in plasma and other tissues were similar after all three routes following the absorptive phase. The percent dose found in the whole blood, red blood cells, and plasma was similar for all doses given by these routes. Less than 0.1-1/2 of the administered dose was recovered as (14)CO2 by any route at 24 hr after administration. Levels in all tissues decreased rapidly, but remained high in whole blood. The red blood cells were found to be the major site of body burden of cacodylic acid.