The effects of moderate intensity exercise of different duration on the cancellous and cortical bone mass of the tibia were investigated in mature ovariectomized rats by bone histomorphometry. A total of twenty 23-week-old female Wistar rats were used in this study. Ovariectomy (OV) was performed on 15 animals, and they were divided into three groups of five animals each: an OV group, an OV + exercise for 30 min a day (EX30) group and an OV + EX60 group. The other 5 animals served as a sham-operated (SH) control group. The exercise consisted of treadmill running at 16 m/min and was performed 5 days a week for 12 weeks after the operation. After 12 weeks of exercise, bone histomorphometry was used to evaluate the cancellous bone (secondary spongiosa) of the proximal tibia and the cortical bone of the tibial shaft. The OV group showed a significant decrease in cancellous bone volume, with increased resorption and formation, compared to the SH group (P < 0.01). Although the OV + EX30 group showed a significant increase in cancellous bone volume, with decreased resorption and increased osteoblastic activity, compared to the OV group (P < 0.05), cancellous bone volume in the OV + EX30 group remained significantly lower than that in the SH group (P < 0.05). There was not a significant difference in cortical bone area between the OV group and the SH group, whereas the OV + EX30 group showed a significant increase in cortical bone area, with decreased resorption on the endosteal surface, compared to the OV group (P < 0.05). There were no significant differences in cancellous bone volume or cortical bone area between the OV group and the OV + EX60 group. The data suggest that moderate intensity exercise of appropriate duration may attenuate ovariectomy-induced cancellous bone loss and increase cortical bone mass in mature ovariectomized rats.