A method for the study of physiological events during transurethral resection of the prostate (TUR) is described. Measurements of volumetric irrigating fluid balance, blood loss, central venous pressure (CVP) as well as blood haemoglobin and serum levels of sodium and glycine were performed at 10-min intervals. This regular interval monitoring (RIM) method offers the possibility to retrospectively correlate changes that occur during TUR at discrete time intervals. In 20 patients undergoing TUR, irrigating fluid absorption occurred throughout the procedure although the risk of having an absorption increased 30 min after surgery commenced. Increase of blood loss coincided with intravascular but not with extravascular absorptions. If there was a rapid massive intravascular absorption of about 500 ml/10 min, the CVP increased greater than 2 mmHg. The total intraoperative blood loss was not greater in patients with large absorption volumes. RIM will detect absorption greater than the urine excretion in any given period of time. If absorption occurs during a limited time of the TUR, RIM allows a partial correction for the urinary excretion in the volumetric fluid balance.