Material strength, as approximated from bending studies of fresh femurs, grew during 1-8 wk of hypergravity at rates which were not measurably affected, even though the animal''s body growth was measurably slower. As a result, femurs supported greater ultimate loads, moments and stresses when compared to control bones of comparable rotational moments of their cross-sectional areas. Chronic centrifugation simulated 3.1 g for 45 male, Swiss Webster mice compared to 37 control animals. Effects were most noticeable after the 1st wk of exposure and for younger animals. Effects were only about 50% that noted in comparable treatment of rats. Four times more mice than rats may be necessary for space experiments designed to test the effects of weightlessness.