The present study has been performed to elucidate the mechanisms of volume regulation in isolated perfused liver. Reduction of extracellular osmolarity by 80 mOsm/L leads to a release of potassium and a sustained alkalinization of effluent. Reexposure to isotonic perfusate leads to reuptake of potassium by the liver and acidification of effluent. Part of the alkalinization could be due to release of bicarbonate parallel to potassium release. Carboanhydrase inhibition and replacement of bicarbonate/CO2 by HEPES buffer, however, do not significantly modify volume regulatory potassium release or reuptake. Reduction of perfusate chloride to 37 mmol/L by replacement of NaCI with raffinose leads to a decrease of liver weight indicative of shrinkage of liver cells. Subsequent omission of 180 mmol/L raffinose leads to potassium and chloride release and to alkalinization of effluent. Volume regulatory release of potassium is impaired in 1 mmol/L quinidine, 1 mmol/L SITS and 5 mmol/L barium. Volume regulatory reuptake of potassium is impaired by 1 mmol/L amiloride. Volume regulatory release of potassium is not appreciably affected by either; 1 mmol/L furosemide, 1 μmol/L verapamil, 1 mmol/L amiloride or 1 mmol/L barium and volume regulatory potassium reuptake proved insensitive to 1 mmol/L furosemide or 1 mmol/L barium. The data suggest that the cells release potassium and chloride during regulatory volume decrease by quinidine, SITS and weakly barium-sensitive transport systems and that regulatory volume increase is accomplished by activation of Na/H exchange.