The aim of the present study was to examine the effect of therapeutic doses of glucocorticoids on the mechanical strength of rat femora. Groups of rats were treated with a glucocorticoid--methylprednisolone (Solu-Medrol)--1 mg/kg/day for 5, 10, 30, and 90 days. One group served as intact control, two control groups were injected with saline for 30 and 90 days and another group of rats had restricted access to food so that their weight gain was reduced to the same extent as the group treated with glucocorticoid for 90 days. The strength of the femora was analyzed by means of a materials testing machine. No differences were found in the short-term treated groups compared to the control groups, but in the group treated with glucocorticoid for 90 days, a reduction in the bending strength of the rat cortical bone was found. Furthermore, this reduction in strength was found after correction for the reduced thickness of cortical bone in the glucocorticoid-treated rats. The results could not be explained solely by the fact that glucocorticoid-treated rats had smaller bones. No alterations were found in bone density or bone ash weight relative to dry weight. The data indicate that the reduction in bone strength induced by glucocorticoids is not only caused by a reduction in bone quantity, but also by a decrease in bone quality.