One hundred female Sprague-Dawley rats, having an initial body weight of 92 to 108 g, were subdivided into 10 equal groups and treated with different hormones. Irrespective of hormone treatment, the animals all received 100 ug/ day of dihydrotachysterol (AT-10) in 0.4 ml of sesame oil, once daily, by gavage through a rubber stomach tube. The experiments indicated that the toxic effects of dihydrotachysterol, especially the loss in body weight as well as the calcification of aorta, heart and kidneys, increased with concurrent treatment with estradiol, cortisol-acetate, cortico-trophin, and thyroxine. Conversely, methyltestosterone and somatotrophin exert a protective effect against AT-10. In these experiments methyltestosterone was as effective as somatotrophin in counteracting the catabolism normally produced by AT-10 and in preventing the atrophy of the epiphyseal growth-cartilage plates.