1. One hundred and sixty-seven mice had cavities drilled into their right femora and were divided into seven groups with or without intrafemoral placement of hormones or placebos.

2. Mice which had cavities drilled only into their femora or had cholesterol placebos or testosterone propionate pellets implanted intrafemorally showed little endosteal bone proliferation in the distal metaphyses and callus formation at the drilled sites four weeks after implantation.

3. Both intrafemoral and systemic applications of ninety micrograms of estradiolbenzoate induced localized hyperostosis in the traumatized regions. However, four weeks after hormone implantation the distal metaphyses of both femora were endosteally proliferated to approximately the same degree, due to the systemic effects of estradiol benzoate.

4. Over eighty micrograms of intrafemorally placed testosterone propionate was needed to inhibit the effects produced by ninety-microgram pellets of estradiol benzoate placed subcutaneously. However, the androgen inhibited the estrogenic activity not only in the drilled femur, but also in the contralateral one.

5. No sexual differences were observed in response to the dosages of hormones given. Differences in femoral changes between castrated and non-castrated mice were not seen.