A cotransport system for Na+, K+ and Cl- in Ehrlich cells is described. It is insensitive towards ouabain but specifically inhibited by furosemide and other 'high ceiling' diuretics at concentrations which do not affect other pathways of the ions concerned. As the furosemide-sensitive fluxes of these ions are no affected by changes in membrane potential, and as their complete inhibition by furosemide does not appreciably alter the membrane potential, they appear to be electrically silent. Application of the pulse-response methods in terms of irreversible thermodynamics reveals tight coupling between the furosemide-sensitive flows of Na+, K+ and Cl- (q close to unity for all three combinations) at a stoichiometry of 1: 1 : 2. The site for each of the ions appears to be rather specific: K+ can be replaced by Rb+ but not by other cations tested whereas Cl- can be poorly replaced by Br- but not by NO(-)3, in contradistinction to the Cl(-)-OH- exchange system. The cotransport system appears to function in cell volume regulatin as it tends to make the cell swell, thus counteracting the shrinking effect of the ouabain-sensitive (Na+, K+) pump. The experiments presented could not clarify whether the cotransport process is a primary or secondary active one; while incongruence between transport and conjugated driving force seems to indicate primary active transport, it is very unlikely that hydrolysis of ATP supplies energy for the transport process, since thre is not stimulation of ATP turnover observable under operation of the cotransport system.