Dry weight evaluation is generally made from clinical observation of body weight (BW) changes, edema, blood pressure, and chest radiograph. In fact, 25-50% of patients on chronic hemodialysis had an incorrectly determined dry weight. To predict dry weight, twenty stable patients on regular hemodialysis were enrolled to investigate the correlation among dry weight, hematocrit, blood volume (BV), and vasoactive hormones including plasma renin activity (PRA), aldosterone (PA), and cyclic 3',5'-guanosine monophosphate (cGMP) values. BV was estimated by an infrared light reflection method. PRA, PA, and plasma cGMP were determined by commercial radioimmunoassay kits. The results showed significantly decreasing plasma cGMP values toward the end of hemodialysis compared with before hemodialysis (15.76 +/- 3.56 pmol/ml vs 33.57 +/- 3.98 pmol/ml; p < 0.05). A significant correlation exists between changes in plasma cGMP values and BV (p < 0.05). In addition, no significant correlation exists between changes in plasma cGMP and BW. A good correlation was found between changes in BV and hematocrit throughout dialysis (r = -0.774; p < 0.001). PRA and PA values predict neither BV nor BW changes. All patients were treated to attain a further ultrafiltration of 0.5 to 1.0 L after reaching dry weight, and we found that the critical point in blood pressure drop occurred when BV decreased by 8% or when plasma cGMP values decreased by 50% from their initial values. Continuous BV monitoring with infrared light reflection and detecting of cGMP throughout hemodialysis could help predict dry weight and avoid dialysis hypotension.