Abstract: A device for flushing the urinary bladder (1) has an instrument inserted from the outside into the inside of the bladder and a feed line (17a, 17b, 10) from a supply receptacle (16) feeding pressurized flushing liquid and a drain line (11, 15) to a drain. The interior bladder fluid pressure is regulated by a pressure regulator connected through the feed line to the bladder. The regulator is held at constant height. The pressure regulator has a valve (31, 32) controlling the flow cross-section which is in turn controlled by the pressure sensor (27) located on the side of the valve facing the bladder and ascertaining the pressure in the feed line (17b, 10). The feed conduit between the regulator and the bladder has a constant flow resistance smaller than the minimum possible flow resistance of the drain conduit multiplied by a maximum pressure variation P.sub.Fmax and divided by a maximum allowable bladder pressure P.sub.Bmax.