To investigate the effect of acute changes of extracellular osmolality on whole body protein and glucose metabolism, we studied 10 male subjects during three conditions: hyperosmolality was induced by fluid restriction and intravenous infusion of hypertonic NaCl [2-5%; (wt/vol)] during 17 h; hypoosmolality was produced by intravenous administration of desmopressin, liberal water drinking, and infusion of hypotonic saline (0.4%); and the isoosmolality study consisted of ad libitum oral water intake by the subjects. Leucine flux ([1-13C]leucine infusion technique), a parameter of whole body protein breakdown, decreased during the hypoosmolality study (P < 0. 02 vs. isoosmolality). The leucine oxidation rate decreased during the hypoosmolality study (P < 0.005 vs. isoosmolality). Metabolic clearance rate of glucose during hyperinsulinemic-euglycemic clamping increased less during the hypoosmolality study than during the isoosmolality study (P < 0.04). Plasma insulin decreased, and plasma nonesterified fatty acids, glycerol, and ketone body concentrations and lipid oxidation increased during the hypoosmolality study. It is concluded that acute alterations of plasma osmolality influence whole body protein, glucose, and lipid metabolism; hypoosmolality results in protein sparing associated with increased lipolysis and lipid oxidation and impaired insulin sensitivity.