1. Extracts of urine were prepared from salt-depleted and salt-loaded subjects by gel filtration of freeze-dried urine on Sephadex G-50. The extracts were dissolved and adjusted to physiological pH and electrolyte concentration.
2. Separated renal tubule fragments were prepared from rabbit kidney cortex and their intracellular sodium and potassium measured after incubation in the dissolved adjusted extracts. Those concerned with the measurement of intracellular sodium and potassium did not know the identity of the samples.
3. Extracts of urine from salt-loaded subjects inhibited the sodium and potassium transport of the tubules whereas extracts of urine from salt-depleted subjects had no effect.
4. Of ten known hormones that were tested, none had any inhibitory effect on the sodium transport of the tubule fragments.
5. It is concluded that the oral intake of large amounts of salt in normal subjects causes the appearance of an unknown substance in the urine which inhibits sodium transport in separated renal tubule fragments.