The "Tractatus de urinis" by Giovanni Maria Lancisi (1654-1720), dated 1696, is held by the Biblioteca Lancisiana, (the Lancisi Library, founded in 1714). It is a short manuscript devoted to the discussion of the physiological mechanisms of urine formation and excretion. It has never before been studied or published. The manuscript was probably written to be used as a didactic text for a university course at the University of Rome "La Sapienza". The analysis of Lancisi's text confirms the importance of the "iatrochemical" pattern of Italian medicine, which used the fermentation pattern to move away from a too strictly anatomical-mechanistic theory, and suggests a response to the debate on the existence of an experimental practice in Rome during the Early Modern Age.