One mg of testosterone propionate in olive oil was injected subcutaneously to female rats on the 5th day after birth. The animals which showed persistent vaginal cornification were thyroidectomized at the age of 10 to 12 weeks. Leukocytic and epithelial vaginal smears appeared within 1 to 2 days after the operation. Irregular cyclicity with the pattern of 2 to 6 days vaginal diestrus and 3 to 10 days estrus continued for as long as a month. Histological examination confirmed the corpora lutea formation in the ovaries of androgen-sterilized+thyroidectomized rats, but did the exclusive existence of vesicular follicles in the ovaries of the androgen-sterilized controls. These results suggest a rapid enhancement in the LH release in androgen-sterilized rats following thyroidectomy.