The serum sodium concentration (S-Na) and the volume of irrigating fluid absorbed were measured during 10-min periods in the course of 85 transurethral resections of the prostate (TUR). The hyponatraemic response to absorption of the irrigant was found to be dependent on the volume of fluid absorbed and the time required for the absorption. During the first 10 min of absorption the distribution volume of the irrigating medium was roughly equal to the extracellular space, but after this period the volume was greater. The sodium level could be compensated for further absorption to the extent of 200-300 ml irrigant per 10-min period; when this volume was exceeded, hyponatraemia was aggravated. The results indicate that in the development of the very low S-Na level typical of a severe TUR reaction, a rapid massive absorption is a more important factor than a large total absorbed volume.