The long-term skeletal effects of ovariectomy and aging were studied in female Sprague-Dawley rats sacrificed at 270, 370, and 540 days after bilateral ovariectomy (OVX) or sham surgery at 90 days of age. The proximal tibia was processed undecalcified for quantitative bone histomorphometry. For continuity, data from these late time points were combined with previously published data from earlier time points (0-180 days). A biphasic pattern of cancellous bone loss was detected in the proximal tibial metaphysis of OVX rats. An initial, rapid phase of bone loss out to 100 days was followed by an intermediate period of relative stabilization of cancellous bone volume at the markedly osteopenic level of 5-7%. After 270 days, a slow phase of bone loss occurred during which cancellous bone volume declined to 1-2%. Both the initial, rapid phase and the late, slow phase of bone loss in OVX rats were associated with increased bone turnover. In control rats, cancellous bone volume remained constant at 25-30% out to 270 days (12 months of age), then decreased to approximately 10% by 540 days (21 months of age). This age-related bone loss was also associated with increased bone turnover. It is interesting to note that the proximal tibial growth plates were closed in approximately a quarter of the control rats by 15-21 months of age. Our data indicate that a slow rate of bone loss and increased bone turnover persist in OVX rats during the later stages of estrogen deficiency. Therefore, the development of osteopenia is coincident with increased bone turnover in OVX rats as well as in aged, control rats.