The effects of progesterone on cancellous bone in young rats were evaluated. Fifty female rats aged 90 days were divided into 5 groups: (1) Sham-operated controls (SHAM); (2) oophorectomized (OVX) rats; (3) OVX rats treated with estrogen (OVX+E); 4) OVX rats treated with progesterone (OVX+P); and (5) OVX rats treated with estrogen and progesterone (OVX+EP). After surgery, some animals were treated subcutaneously (s.c.) with 0.5 mg of estradiol dipropionate depot, with 5 mg of hydroxyprogesterone capronate depot, or with both hormones, at intervals of 7 days. After 180 days, the bone mineral density (BMD) and bone histomorphometry of their lumbar vertebrae and serological parameters were measured. In the OVX+P group, the BMD and the bone volume as shown by bone histomorphometry were slightly higher than in the OVX group, but not significantly. These parameters in the OVX+E and OVX+EP groups were significantly higher than in the OVX group. The OVX+P group showed a significant increase in the labeled surface and a decrease of the osteoclast number, compared to the OVX group. The OVX+P group showed increases of serum alkarine phosphatase and osteocalcin. From these results, it can be concluded that progesterone treatment might intensify bone formation and suppress bone resorption on cancellous bone in oophorectomized rats, although progesterone alone does not have a positive effect on BMD and bone volume.