Several investigators have studied the mechanical properties of long bones of rats and have found that bone strength may be associated with bone mineral content. In this study we examined further the interrelationships of whole bone properties and bone ash weight of the rat femur. Using a 3-point bending test, the bones were evaluated during lactation and after one to three pregnancy + lactation periods--experiments in which bone ash was significantly reduced. A relationship was established between the time of lactation and the ash weight, stiffness, strength, and toughness but not the ductility. Using all of the experimental data, a highly significant relationship was observed between ash weight and stiffness. A similar significant correlation was revealed between bone ash and strength or toughness, although drawing individual lines for each experiment rather than a common line for all the experiments appeared more appropriate. Given the inherent inaccuracies in the material property measurements of bone, the results suggest that routine measurements of whole bone properties provide an important and sensitive way to evaluate bone quality and that these properties correlate significantly with the bone ash weight.