1. Phenol red and neutral red are excreted by the perfused frog's kidney by different routes.
2. Phenol red is excreted chiefly through the glomerulus, neutral red through the tubules.
3. Some slight excretion of each of these dyes by the converse mechanism is possible, though there is no evidence in our experiments that necessitates such a conclusion.
4. The importance of methods leading to the production of a normal volume of urine by the perfused kidney is emphasized.