SUMMARY The effect of complete destruction of the thyroid gland by radioactive iodine on reproduction in the female mouse has been examined in detail. Possible direct effects of radiation on reproduction were differentiated from those due to thyroid destruction. Destruction of the thyroid tissue in the male mouse had no effect on fertility. In the thyroid-deficient female both fertility and litter frequency were unaffected, but the oestrous cycle was prolonged and the duration of 46% of gestations was longer than normal. There was a general shift in the distribution of litter size towards small litters in the thyroid-deficient group which reduced the average number of young per litter. Lactation, as judged by the weight of the offspring at weaning, was not affected by complete absence of the maternal thyroid gland. After weaning the growth rate of offspring of either sex from both thyroid-deficient and normal mothers was the same. Although the pituitary glands were enlarged in all the thyroid-deficient animals examined, tumours were not observed.