In the present study, dioctyltin chloride (DOTC: 100mg/kg, BW) was orally administered to immature (30-day-old) male rats, and the acute toxic effects were studied. Di- and monooctyltin (its metabolite) accumulations were mainly detected in the liver, and peaked 48h later. A similar pattern was also found in the kidney, but the levels were low or trace amounts. Significantly low thymus and spleen weights were detected in DOTC-treated animals. Increased apoptotic cell numbers in the thymus and spleen were observed in DOTC-treated animals also. Although the expression of 97 genes involved in apoptosis was studied in the thymus, at least 24h after treatment, we could not detect clearly different expressions between DOTC- and vehicle-treated animals. The present results suggest that DOTC was selectively immunotoxic. One of the mechanisms for its immunotoxicity would be via its stimulation of immune cell apoptosis.