In contrast with the striking suppressing effects of triiodothyronine and thyroxine on the lesions produced by aminoacetonitrile in rats, the lesions produced by [beta]-aminopropionitrile were only slightly suppressed by these hormones. The activity of eight thyroxine analogues in the suppression of the aminoacetonitrile lesions in rats was found to be far below that of triiodothyronine. Corticosterone and cortisone suppressed only slightly the lesions produced by aminoacetonitrile in rats.