Direct photon absorptiometry was used for the in vivo measurement of bone mineral mass in the rabbit tibia. Bone mineral mass in 124 male and female rabbits was correlated (p less than 0.001) with age to 26 weeks and weight to 5.5 kg. Examining reproducibility and repositioning error, three rabbits were measured 12 times each. Bone mineral mass in the three rabbits was: 0.432 +/- 0.020 g/cm2, 0.414 +/- 0.010 g/cm2 and 0.487 +/- 0.037 g/cm2. The greatest coefficients of variance observed for measurements without repositioning and measurements with repositioning were 4.8% and 7.6%, respectively. The technique provided sensitive, reproducible and noninvasive measurement of bone mineral mass without anesthesia or undue restraint.