SUMMARYThis study extends permeability (P) data on chloride, urea and water in red blood cells (RBC), and concludes that the urea transporter (UT-B) does not transport water. cms -1 (human). DIDS, DNDS and phloretin inhibit P Cl by >99% in all RBC species. PCMBS, PCMB and phloretin inhibit P urea by >99% in Amphiuma, dog and human RBC, but not in chick and duck RBC. PCMBS and PCMB inhibit P d in duck, dog and human RBC, but not in chick and Amphiuma RBC. Temperature dependence, as measured by apparent activation energy, E A , of P Cl is 117.8 (duck), 74.9 (Amphiuma) and 89.6kJmol −1 (dog). The E A of P urea is 69.6 (duck) and 53.3kJmol −1 (Amphiuma), and that of P d is 34.9 (duck) and 32.1kJmol −1 (Amphiuma). The present and previous RBC studies indicate that anion (AE1), urea (UT-B) and water (AQP1) transporters only transport chloride (all species), water (duck, dog, human) and urea (Amphiuma, dog, human), respectively. Water does not share UT-B with urea, and the solute transport is not coupled under physiological conditions.