The purpose of this study was to assess the long-term effects of aging and sex hormone deficiency on skeletal metabolism and body composition in rapidly growing male rats. Sprague-Dawley male rats were sham-operated (sham) or orchidectomized (ORX) at 3 months of age. Eight sham rats and eight ORX rats at each time point were serially sacrificed at 3, 4, 8, 12, 15, and 23 months of age. Bone mass in sham rats rapidly increased until 8 months of age, then slightly increased between 8 to 12 months of age; thereafter, an age-related decrease in bone mass was found between 12 to 23 months of age. In sham rats, bone formation parameters decreased between 3 and 8 months, and maintained at the lower level between 8 and 23 months of age, while bone resorption parameters decreased between 3 and 12 months, and thereafter, increased with age between 12 and 23 months of age. ORX significantly inhibited age-related gain in body weight, lean body mass, and cancellous and cortical bone mass and decreased peak bone mass (approximately 20% less versus sham). Further, we found that the lower bone and lean body mass in ORX rats was due to the lack of age-related gain rather than the net loss from basal controls. These data suggest that sex hormones are important factors for the accumulation of peak bone and lean body mass in male rats.