1. 1. The diuretic furosemide, when added to the outside solution at a concentration of 5 · 10-4 M, increases the electrical potential difference (PD) across the isolated frog skin, but the short-circuit (Isc) is unchanged. Lower concentrations had no significant effect on these electrical parameters. 2. 2. When SO4 2- or NO3 - are substituted for Cl- in the Ringer's solution furosemide has no effect on the PD or Isc. 3. 3. Simultaneous unidirectional fluxes of Na+ and Cl- show that furosemide (5 · 10-4 M outside) reduces both the influx and outflux of Cl-, while the Na+ fluxes are not altered. 4. 4. Furosemide (5 · 10-4 M) on the corium side of the frog skin had no significant effect on either PD, Isc or unidirectional fluxes of Cl-. 5. 5. It is suggested that furosemide reduces passive Cl- transfer, possibly by interacting with the Cl-/Cl- exchange diffusion mechanism which has been observed in this tissue. These observations further suggest that perhaps the diuretic action of furosemide may be mediated by such an effect on passive Cl- permeability which is linked to the active Cl- transport mechanism in the renal tubule.