Groups of immature female albino mice were maintained at 24[degree], 30[degree] and 35[degree] C. Certain groups were given thiouracil or small amts. of thyroprotein incorporated in their ration. In mice kept at 24[degree] C. ovary wts. were increased by 0.0125% thyroprotein but depressed by higher levels. At 30[degree] and 35[degree] C, 0.005% thyroprotein had no effect on the ovary wt. Thyroprotein at all temps. and all levels employed caused significant increases above their controls in uterus wt. Both thiouracil and thyroprotein admn. resulted in earlier opening of the vagina. The thiouracil-fed mice showed continuous estrus, as judged by vaginal smears. Slightly hyperthyroid mice exhibited regular estrous cycles. Sexual maturation was accelerated at all levels of thyroprotein feeding as shown by the occurrence of corpora lutea and progestational changes in the uterus. The thiouracil groups had ovaries packed with follicles, but no corpora lutea, together with typical estrogen stimulation of the uterus.