1. The ability of chloride and phosphate to support urine formation has been studied under different conditions. When sulphate is used as a balancing anion, rate of urine formation is linearly related to the external chloride or phosphate concentration.
2. Tubules can concentrate phosphate in the urine by a mechanism which is independent of other anions. Phosphate U/P declines with an increase in external phosphate concentration.
3. In the absence of phosphate, chloride is slightly concentrated in the urine. Chloride U/P is reduced by phosphate but unaffected by sulphate.
4. When an extensive series of anions was tested, rate of urine production was related to the size of the anions. Phosphate was the only exception because despite its large size it supported the highest rate of urine production.
5. Arsenate competitively inhibits phosphate transport, resulting in an augmentation of chloride transport. Copper, however, blocks urine formation in the presence of chloride but not in the presence of phosphate.
6. The experiments indicate two separate mechanisms for anion transport. Anions such as chloride probably pass through pores with an estimated radius of 3.6 Å., whereas phosphate is transported by a carrier.