In order to determine the effects of exercise on the calcium status of selected axial and appendicular bones of mature rats, female Sprague-Dawley rats (8-9 mo.) were divided into three groups including, two months (E2, n = 8) or four months (E4, n = 9) of exercise, and four month sedentary controls (S, n = 10). Exercise consisted of treadmill running for 1 hr/day, 5 days/wk at a speed of 14.1 m/min and 8 degrees elevation. After sacrifice all femurs, tibia/fibula complexes, ribs (T7), and vertebrae (T7) were excised, cleaned, weighed and measured for length and volume. After freeze-drying and bone hydrolysis in 5N HCl, total bone calcium contents and concentrations were determined spectrophotometrically. The acid soluble, appendicular bone calcium contents of the E4 group were significantly greater than S for the femur and tibia respectively: E4 = 159.78 +/- 3.44 mg (mean +/- SEM), 129.46 +/- 4.87 mg; S = 140.03 +/- 5.04 mg, 110.40 +/- 4.71 mg. Bone calcium concentration (mg/g dry bone) also was significantly greater in the tibia/fibulas, ribs and vertebrae of the E4 group than the S group. With respect to other training-induced effects, the oxygen carrying capacity of the blood, as well as the heart and lung DNA and protein concentrations did not change after four months of exercise training. Within four months, moderate exercise can increase the calcium deposition in the bones of mature, female rats.