The capacity of the male urethra in patients and on anatomical preparations has been measured. Because measurement of capacity in the living proved to be strongly influenceable, the capacities determined from anatomic outlet preparations represent the real values in practice. The average value from a group of 33 corpses was 10.58 ml. In urologic practice, the capacity of the urethra assumes particular importance with catheterization. Optimal filling of the urethra appears to be important in order achieve maximum performance for the sliding catheter. Additionally, the filling material should have the corresponding viscosity and an anesthetizing effect.