The influence of variations in fluid state on diurnal blood pressure was studied by measuring day-time and night-time blood pressure during a 3-day interdialytic period in 10 normotensive and 10 hypertensive haemodialysis patients using Spacelab 90207 Monitors. Ambulatory blood pressure was also measured during 24 h in 11 normotensive and nine hypertensive CAPD patients, and in nine normotensive and 11 hypertensive control patients with a normal renal function. Antihypertensive drugs had been discontinued for at least 3 weeks before the study period. Optimal dry weight in the haemodialysis patients was estimated by echography of the inferior vena cava and in the CAPD patients on clinical grounds. Although in the dialysis patients and controls a significant nocturnal blood pressure reduction was found, day-night blood pressure difference in the dialysis patients was blunted when compared with the control patients. No significant differences in diurnal blood pressure variation was found between the normotensive and the hypertensive patients. Day-night blood pressure differences in the haemodialysis patients did not change during the 3-day interdialytic period. Also the more stable fluid state of the CAPD patients was not associated with significant different diurnal blood pressure variation compared to the haemodialysis patients. We conclude that factors other than changes in extracellular fluid volume are responsible for a blunted day-night difference in blood pressure in dialysis patients.