There are a few scattered publications on primary tubulopathies, where a net secretion of phosphorus by renal tubules, a mechanism usually not existing in man, is shown. By means of a further case, we are able to demonstrate a 30% increase of phosphate excretion over filtration after a phosphate load. The mechanism of action will be discussed. Under certain conditions like a rapid increase in serum phosphorus or after volume expansion a tubular secretory mechanism of phosphate can be unmasked. As this secretory mechanism goes into action only beyond a certain serum threshold of phosphorus and a transport maximum cannot be shown it is most likely that passive transport by means of an increased diffusion cell to lumen is the proposed mechanism involved.