123 transurethral resections of the prostate were studied. The operations were randomized to either continuous or intermittent technique. The first 64 operations were performed using an irrigating fluid pressure head of 65 cm (measured from the top of the operating table). In the rest of the operations an 80 cm pressure head was used. In this way four groups were obtained comprising 30 to 31 patients, differing from each other in respect of irrigating technique. Isotonic 5% mannitol solution was used as an irrigating fluid. The irrigating fluid absorptions were calculated from the plasma mannitol levels determined immediately postoperatively. The use of a trocar significantly lessened the average absorbed fluid volume, while there was no significant difference between the groups of high and low irrigating fluid pressure head. The intraoperative bleeding, the decrease in serum sodium, the resection time and resection rate did not differ significantly between the four groups.