Taurine is a cerebral osmoprotective molecule during chronic hypernatremic dehydration. In these experiments, we investigated the role of taurine in osmoregulation during acute hyponatremia. Taurine deficiency was induced in experimental cats (n=6) by feeding a taurine-free diet for 8-10 wk, whereas control counterparts (n=6) consumed a regular diet. Hyponatremia was provoked in all cats over 54 h by daily injections of 5% dextrose in water (7.5% body wt) and vasopressin (20 U/d). The serum Na+ concentration was abruptly lowered to 110 ± 3 and 117 ± 2 mmol/L, in experimental and control animals, respectively. The cerebral total and intracellular water compartment sizes were reduced from 486 ± 11 to 441 ± 11 ml/100 g dry wt and from 357 ± 7 to 309 ± 12 mL/100 g dry wt, respectively, in control versus experimental cats, p<0.05. There was a significant linear relationship between cerebral taurine content and the intracellular water compartment size in all animals,p<0.02. Taurine displayed a similar osmoprotective capacity in muscle tissue in these studies. We conclude that taurine is an osmoregulatory molecule in cerebral and extracerebral tissues during severe hyponatremia. Reductions in tissue taurine content may complement decreases in cytosolic electrolyte levels during adaptation to more prolonged hyponatremia.