Four groups of rats were fed a low-calcium diet (0.02 %) and two groups a normal calcium diet (0.50 %). After 2 weeks one group from each dietary calcium level was sacrificed and the femurs examined. The remaining three groups which had been kept on the low-calcium diet were then given diets richer in calcium: 0.25, 0.35 and 0.5 %. The remaining group on the normal calcium diet continued receiving it. After two more weeks all animals were sacrificed and their femurs examined. Rats fed the low-calcium diet lagged behind in weight gain; ash content, breaking strength, and modulus of elasticity of their femurs were much lower than those of animals on the normal calcium diet. Feeding diets with increasing calcium content increased bone ash and improved mechanical properties. However, improvement of breaking strength and elasticity did not parallel strictly increases in mineralization. Repletion with a normal calcium diet (0.50 %) failed to increase bone ash to normal levels but mechanical properties were improved by repletion with substantially lower dietary calcium levels. Maximal breaking strength was obtained by repletion feeding with a dietary calcium level which failed to increase bone ash. It seems that factors other than mineralization determine breaking strength and elasticity of bone recovering from calcium deprivation.