1. Methods for the collection of samples of urine from different parts of the nephridium of Lumbricus are described.
2. The osmotic pressures of these samples have been measured by determination of freezing-point depression and have been compared with the osmotic pressure of the medium surrounding the nephridium.
3. The results of this comparison indicate that the ability to form hypotonic urine is certainly present in the wide tube, is possibly present in the middle tube and is probably not present in the narrow tube.
4. The analogy between the nephridium and the vertebrate nephron is discussed.