Cell membranes of various vertebrate cells catalyze a Na+ + K+ + 2Cl- cotransport specifically inhibitable by furosemide and other high ceiling diuretics. The energetics of this process is not elucidated unequivocally. It was clearly shown that cotransport is no ATP-consuming process. We assume that transport is secondary active functionally coupled to the operation of the electrogenic Na+-K+ pump. The role of this transport system in transepithelial ion movement is that it serves as flux amplifier, doubling from 6 to 12 the number of osmotically active particles transported per ATP hydrolyzed. In concert with Na+-K+ pump, cotransport provokes net uptake of KCl into the cell and therefore cellular swelling. This process is regulated by a feedback control system for cell volume; if actual volume reaches reference value, cotransport is switched off to prevent further swelling. How cell volume is measured is not known, nor is the nature of the signal generated to switch cotransport from the operating to the nonoperating state or vice versa. cAMP-level or intracellular Ca2+ play no role as signals or as part of the volume-sensoring mechanism. Theophylline, other alkylxanthines, and some purine ribosides influence cotransport indirectly by reducing reference volume. The role of cytoskeleton in volume regulation is obscure. While high Concentrations of cytochalasin B and of colchicin do not influence cell volume, it is reduced by vinblastine and also by lectins, for example concanavalin A. Volume reduction is accompanied by reduction in cellular KCl content. The observation that during hypertonic incubation protein synthesis is inhibited can be traced back to a correlation between cell volume and protein synthesis and not to elevation of osmolarity per se. Reduction in cell volume under isotonic conditions by varying K+ and/or Cl- concentration or by furosemide inhibition of cotransport is strongly correlated to inhibition of protein synthesis. The reason for this correlation is not yet clarified. Not all cells showing furosemide-sensitive cotransport are able to regulate it, for example lymphocytes. For mammalian erythrocytes drastic species differences exist; while cells from man, rabbit, rat, and mouse all show cotransport, only cells from rat (and mouse?) are able to regulate cotransport.