Absorption of irrigating fluid into the blood during transurethral resection of the prostate is associated with diffusion of sodium ions from the interstitial fluid space into the plasma. Some of this sodium is "trapped" and removed from the body in connection with bleeding and excretion of urine. The magnitude of this sodium loss was calculated over 10-min periods throughout 20 operations during which absorption of glycine solution occurred. The amount of trapped sodium increased with the amount of blood lost, a 1000-ml bleed correlating with a loss of 10 mmol of sodium. Two thirds was trapped with the plasma loss and one-third with the osmotic diuresis. This mechanism contributes to the absolute loss of sodium from the body. The total sodium loss, however, accounts for one third of the maximum hyponatraemia and is still dominated by the plasma-derived sodium excreted during the glycine-induced osmotic diuresis.