Pentavalent inorganic arsenic was introduced by intravenous, intratracheal, gastrointestinal, and skin application in doses 0.1 to 4.0 mg/kg in rats. Isotopic technics were applied by use of As(74). It was found that the dynamics of arsenic distribution in the body as well as the kinetics of its elimination in urine and feces varies very substantially, depending on the mode of administration. Intravenous administration of As causes immediate appearance of arsenic in most tissues and a slow decrease of its concentrations in time. Similar situations could be observed with intratracheal dosing, because arsenic is very rapidly absorbed from the site of administration. Concentration in tissues increases more slowly after gastrointestinal resorption. Skin application causes first the accumulation of arsenic in the skin and next continuous, slow transport from the skin into the blood stream. The rate of skin resorption was 1.14-33.1 mug/cm(2)-hr for 0.01-0.2M concentrations. The red blood cell level of arsenic is very substantial and does not change with time, which indicates the accumulation of arsenic in this tissue. The elimination of arsenic occurred chiefly in urine and feces, but the urine/feces ratio changed very substantially, depending on the route of administration. The kinetics of arsenic elimination in urine was multiphasic, being three-phase in case of intravenous and intratracheal administration and two-phase after gavage and skin resorption. After intravenous administration of As, the half-times of elimination were 2.5, 10, and 690 hr, respectively. Administration of selenium salts during the slow phase increased the rate of arsenic elimination. The straight-line relations found between the absorbed dose of arsenic and its blood or urine concentrations could serve as baselines for exposure tests for humans.