The influence of maturation and senescence on the mechanical strength of bone was studied in rats by measuring the breaking load of the femur shaft and calculating the breaking strength. Strength of cartilage was studied by determining the ultimate load of femoral epiphyseal plate. A sharp increase of all mechanical parameters was found between the age of 1 and 4 months which was attributed to a maturation process. The maximum was achieved at 1 year followed by a decrease in all parameters. This was considered to reflect the aging or senescence process. The following biochemical parameters were determined in the same specimens: collagen and its soluble fractions, glycosaminoglycans, the fractions thereof, and elastin. The content of insoluble collagen was very closely correlated to the strength data, thus demonstrating an increase between 1 and 4 months, a plateau up to 1 year and a decrease at 2 years. During the maturation process, the glycosaminoglycans, especially the chondroitin sulfates, fell considerably. But also during the aging period, a further decrease was noted. The elastin content fell only slightly during the whole life span. The excellent correlation between strength parameters and content of insoluble collagen as found previously in skin could thus be confirmed. Strength of bone is apparently not dependent on the content of glycosaminoglycans and elastin. This indicates the major role of insoluble collagen for the mechanical properties of connective and supporting tissue. The studies presented here show that in rat bone the senescence process can be separated from the maturation process.