Sorbitol production in the renal medulla increases in dehydrated rats, indicating that aldose reductase activity varies with the state of hydration. This response could be due to an increased synthesis of the enzyme (Moriyama T et al. J Biol Chem 1989:264:16810-16814) and/or a change in aldose reductase activity caused by acute changes in intracellular ionic composition, ionic strength, osmolality, or pH. Aldose reductase activity in tubules dissected from kidneys of control rats and rats undergoing water diuresis was measured, and the tubules were permeabilized so that changes in intracellular composition that would occur during dehydration could be induced experimentally. Aldose reductase activity did not change consistently as sodium, potassium, chloride, or osmolality were varied. Aldose reductase activity did increase acutely when sulfate was raised or when pH was lowered to pH 6.2 to 6.8, corresponding to the pH optimum of the enzyme. The small magnitude of change in enzyme activity suggests that the major influence of dehydration on aldose reductase activity is to increase enzyme synthesis. It was concluded that aldose reductase activity is not acutely regulated by changes in sodium, potassium, chloride, or osmolality. The stability of aldose reductase activity despite changes in ionic composition or osmolality supports the hypothesis that acute regulation of intracellular sorbitol content occurs by variation in cell sorbitol permeability and not by variation in cell sorbitol production.