1. Uptake of a number of isotopically labelled extracellular tracers by rat liver in vivo was studied.2. A residual intravascular space of 3.0-3.5% following exsanguination was measured by use of [(51)Cr]red cells. This space was significantly greater than that of skeletal muscle and was also greater in nephrectomized as compared to intact rats.3. [(131)I]albumin and polyvinylpyrolidine (PVP) were found to distribute initially in a space of 6-7% and this was followed by a delayed rise that was greater for PVP. The extravascular portion of this space is termed the large-molecule accessible space and is believed to correspond to the histological space of Disse.4. [(14)C]sucrose and inulin initially distribute in a space of 10-12% followed by a continued late increase. This initial space is termed the small molecule accessible space and is thought to include most of interstitial space.5. (36)Cl(-) and stable Na(+) and Cl(-) spaces were constant at 19-21% and can be used to put an upper limit on extracellular space.6. Operationally, the 1 hr inulin space in the nephrectomized rat was felt to be the best indicator of extracellular space.7. Total electrolytes were measured and intracellular values of 22, 165 and 22 m-mole/l. cell water for Na(+), K(+) and Cl(-), respectively, were calculated.8. By use of [(14)C]DMO, an intracellular pH of 7.25 and an intracellular HCO(3) (-) of 13 m-mole/l. cell water were calculated.