The extracellular fluid space in dog bone has been examined using a series of isotopically labelled compounds. Sodium-77 bromide and indium-113m ethylenediaminetetracetic acid were used as extracellular fluid space markers, radioactive water as a total fluid space marker, and potassium-43 chloride to examine the existence of a bone membrane. The clearance of each tracer from bone was monitored for a period of 2h post-injection. Graphical analysis of the clearance curves shows that the number of exponential functions vary depending on the type of tracer used. The fact that a sum of three exponential terms can completely describe each curve indicates that a simple model consisting of three compartments is sufficient to approximate the clearance of these tracers from bone and its associated fluid space. It is concluded that bone consists of an extracellular fluid space, and that this space may well play an important part in the transference of solutes and the mechanisms involved in their localization on the hydroxyapatite crystals of bone.