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Regulation of body water, plasma volume (PV), hematocrit (HCT) and plasma. concentration of protein, C1, Na+ and K+ was studied in ground squirrel (Citellus tridecemlineatus) as a function of dehydration. Dehydration from water deprivation was characterized by a reduction of PV, and an increase in + he HCT. Plasma C1 and Na+ in water deprived squirrels tended to increase 11.70/,; and 5,1 % but without statistical significance. The rise in the per cent red blood cells and the tendency of Na+ and C1 to increase in the plasma of water deprived squirrels may indicate water loss. from the plasma. Water deprivation had no effect on plasma protein and K+ concentrations. Fat free body water was 73.1% in control squirrels and 7 0.2% in water deprived animals. U/P ratios of C1 and Na+ concentrations were 1.2 and 1.1. Analysis of the distribution of fluid loss in water deprived squirrels show that PV lost 1.4 times more fluid than cellu lar and interstitial spaces.