Using radiogrammetric and photonabsorptiometric methods, we determined changes in bone mass in Wistar rats as a function of age and sex. In the four age groups studied (10-, 18-, 30-, and 78-week-old rats), males displayed greater bone mass than females. With increasing age, bone mass also increases up to the 78th week. However, the rate of this increase was slower in the 78-week age group. Whereas the length and periosteal diameter of the bones studied were increased in old age groups, the endosteal diameter decreased in females and increased in males. The results show that, unlike human bone, rat bone mass persistently increases with aging, and female rats continue to gain bone at the endosteal site. Furthermore, these findings demonstrate that radiogrammetric morphometry and photonabsorptiometry may be used as an experimental tool to investigate bone mass changes in small animals.