Fluid redistribution in isolated perfused cat calf muscle caused by rapid increases in plasma osmolality was studied using NaCl or sucrose. Extracellular tracers (51Cr-labeled EDTA or [3H]mannitol) were added to the perfusate 90 min before solutes were added, and samples were taken from plasma immediately before osmolality was increased and 17, 40, and 65 min later. Interstitial fluid volume (IFV) was calculated as extracellular volume (ECV) minus plasma volume (Evans blue dye). Total tissue water changes (delta TTW) were measured by continuous recording of tissue weight. Change in intracellular volume (delta ICV) was obtained from delta TTW--delta IFV. TTW, IFV, ICV, and plasma osmolality were in steady state after 17 min. Changes in hydrostatic and colloid osmotic pressure were insignificant in comparison with small-molecule osmotic pressure changes. The apparent volume of TTW participating in the fluid shift averaged 65 +/- 1 ml/100 g (SE) over a wide range of osmolality increases. In contrast to the large changes in TTW, IFV was not altered by osmolality. Thus decreases in TTW were similar to cell dehydration. Hence, increases in plasma volume induced by hypertonic fluids may come entirely at the expense of cell volume, not interstitial volume.