A single oral dose of 4080 mg.kg and a single dermal dose of 2500 mg/kg of aamitrole, the highest doses given, were not lethal to adult rats. Dietary levels of 0, 100, 500, 100 and 25 ppm amitrole were fed to groups of 10 male and 10 or 12 female rats. The dietary levels of 1000 and 500 ppm were fed for 107-110 days, the dietary levels of 100 and 25 ppm for 240-247 days to the F0 generation and for 141 days to the F1b generation. At the dietary levels of 100 and 500 ppm the pups that were born were smaller and had atropic thymuses and spleens indicative of runt disease. Most of them had died within a week after weaning. Reproduction was not affected at the lower dietary levels. Hyperplasia of the thyroid was seen in all rats fed dietary levels of 100 ppm or greater. In the 25 ppm group 4/10 F0 and 4/10 F1b females and 6/10 F1b males, but none of the F0 males developed hyperplasia of the thyroid.