1. Experiments were carried out on the abalone, Haliotis rufescens, to discover some of the mechanisms involved in urine formation. Test substances were infused into the blood stream and their concentrations followed in serially taken samples of blood, pericardial fluid and urine from the right and left kidneys.

2. Inulin appears to be filtered since its concentration is essentially the same in samples of blood, pericardial fluid and urine from each kidney. The concentrations of phenolsulphonphthalein and p-amino hippuric acid were considerably higher in right kidney urine than in the other fluids, indicating that this kidney is capable of active secretion.

3. Glucose occurred in lower concentrations in the left kidney urine than in blood and pericardial fluid, suggesting a reabsorption of glucose by this kidney.

4. Dye solutions of T-1824 and other materials infused slowly into the pericardial cavity appeared in the urine of both kidneys, suggesting the presence of two functional reno-pericardial canals; presence of both right and left canals was verified by dissection and observation.

5. From the results obtained it appears that the primary step in urine formation is filtration of blood through the walls of the atria into the pericardial cavity. This fluid then passes via the reno-pericardial canals into the kidneys where on the right side substances may be actively secreted into it and in the left kidney substances may be actively reabsorbed.

This work was done at the Hopkins Marine Station of Stanford University under the sponsorship of Donald P. Abbott and was made possible by fellowship support from the National Institutes of Health.