Fentin hydroxide (triphenyl tin hydroxide) was given in the diet to rats at the rate of 400 ppm. This amount reduced the food consumption and, within 7-34 days, caused death from starvation and hemorrhage in all tested rats. Dietary levels of 100 and 200 ppm reduced food consumption and weight gain, particularly during the first week. By week 7, the animals had adapted to these concentrations of the chemical, and the food consumption was the same as that of the controls. The food intake or weight gain was not affected in rats fed 50 ppm of fentin hydroxide for 276 days. After 64 days of dietary exposure at 200 or 100 ppm, reduced fertility was observed in male rats, but fertility improved later, and was comparable to that of the control rats by day 113. The difference was explained by the partial stavation which the animals suffered, particularly during the first week of the experiment. Gross and microscopic examination of rats fed fentin hydroxide at the rate of 200, 100, or 50 ppm for 276 days did not reveal changes in any organ that could be related to the compound.