The effect of the loop diuretic furosemide (4-chloro-N-furfuryl-5-sulfamoyl-anthranilic acid) on the thiol-dependent, ouabain-insensitive K(Rb)/Cl transport in low K+ sheep red cells was studied at various concentrations of extracellular Rb+, Na+ and Cl-. In Rb+-free NaCl media, 2 X 10(-3) M furosemide inhibited only one-half of thiol-dependent K+ efflux. In the presence of 23 mM RbCl, however, the concentration of furosemide to produce 50% K+ efflux inhibition (IC50) was 5 X 10(-5) M. In Rb+ containing NaCl media, the inhibitory effect of 10(-3) M furosemide was equal to that caused by NO-3 replacement of Cl- in the medium. The apparent synergistic action of furosemide and external Rb+ on K+ efflux was also seen in the ouabain-insensitive Rb+ influx. A preliminary kinetic analysis suggests that furosemide binding alters both maximal K+(Rb+) transport and apparent external Rb+ affinity. In the presence of external Rb+, Na+ (as compared to choline) exerted a small but significant augmentation of the furosemide inhibition of K+(Rb+) fluxes. There was no effect of Cl- on the IC50 value of furosemide. As there is no evidence for coupled Na+K+ cotransport in low K+ sheep red cells, furosemide may modify thiol-dependent K+(Rb+)/Cl flux or Rb+ (and to a slight degree Na+) modulate the effect of furosemide.