In this study we investigated the interactions between volume changes (body weight), plasma atrial natriuretic peptide (pANP) and ambulatory blood pressure (BP) in 10 patients with end-stage renal disease undergoing regular hemodialysis (HD) treatment three times weekly. Most patients retained their diurnal BP variation when their BP was adequately controlled. Interdialytic weight gain was 1.3 +/- 0.2 kg and the day-time systolic BP increased 12.5 +/- 4.8 mm Hg on the second interdialytic day. pANP did not correlate (r = -0.07, p = 0.85) with this BP elevation, but there was a fairly strong positive correlation (r = 0.61, p = 0.06) between interdialytic weight gain and systolic BP. The mean pANP level decreased from 149.7 +/- 18.2 to 117 +/- 0.1 ng/l during HD and continued its decrease to 83 +/- 12.2 ng/l at 20 h after an HD session. The total decrease from 149.7 +/- 18.2 to 83 +/- 12.2 ng/l was statistically significant (p = 0.001). Since the lowest pANP value was found 20 h after completion of the dialysis session, body weight is a more reliable indicator of volume reduction during HD than pANP. The results indicate that in HD patients weight gain between two dialysis sessions increases the day-time systolic BP but not the diastolic BP. Diurnal BP variation is maintained as long as BP is adequately controlled either by volume control or by drug treatment.