Morphologic and modeling parameters were studied in the diaphysis of female rats femora between 60 and 850 days of age. Modeling rates were measured by the vital labeling technique. With the data obtained, a mathematical bone model has been developed that describes the cross section of the femur midshaft by two ellipses, defined by four time-dependent functions of the radii, and one drift function that describes the movement of the whole diaphysis in the transverse direction. With increasing age the apposition(MF), forming(BF), and resorbing(BR) rates decrease continuously from 2.5 mm/year(MF), 850 %/year(BF), and 450 %/year(BR) at 60 days to 0.05 mm/year(MF), 10.6 %/year(BF), and 6.7%/year(BR) at 850 days, whereas the endosteal and periosteal diameters increase. In young animals the fraction of forming and resorbing areas of the periosteal and cortical-endosteal surfaces is comparable, whereas in old rats most of periosteal surface is forming and most of the cortical-endosteal surface is resorbing.