In order to provide a firm basis for the analysis of experiments on the kinetics of ion and solute exchange in bone, the total water space, extracellular space, and vascular (plasma plus erythrocyte) space in the tibial diaphysis in adult dogs were estimated. Tracers were injected intravenously into 40 dogs after ligation of the renal pedicles. Estimates were made of the volumes of distribution of radioactive tracers of varying characteristics (molecular weight, solubility, and charge), by taking samples of blood, plasma, and bone 3 hr after injection. The volumes of distribution (milliliters per milliliter of cortical bone) were: for tritiated water (a total water marker) 0.245 ± 0.003 (mean ± SD); for [¹⁴C]sucrose (an extracellular marker) 0.043 ± 0.001; for ¹¹¹In-labeled transferrin (a plasma marker) 0.008 ± 0.003; and for red blood cells 0.005 ± 0.002. Interstitial fluid space was estimated by subtracting the estimated plasma space from the estimated extracellular space and it was 0.034 ml/ml of bone. Estimates of haversian canal volumes, obtained morphometrically, were 0.015 ml/ml bone and were compatible with the estimates of the vascular space.