We present the aphorisms related to nephrology in The Guide Book or The Book of Aphorisms (Kitāb al-Murshid aw al-Fuṣūl) by Rhazes (865-925 AD), based on the edited version of the book by the noted historian of Islamic Medicine A. Z. Iskandar published in 1961. We briefly discuss the aphorisms related to nephrology presented in the Guide Book under the titles of “diuresis”, “the urine”, “discharge of excretions of the kidneys”, “discharge of excretions of the bladder”. The aphorisms contain basic and general physiological and pathological information on diuresis and urine based on traditional humoral principles. They do not address any specific diseases.
Keywords: Rhazes, Medieval medicine, The Guide Book, The Book of Aphorisms, History of Nephrology