Plasma brain natriuretic peptide (BNP) levels have been reported to increase in patients with heart failure and end-stage renal disease (ESRD); however, little is known about molecular forms of plasma BNP that increase in these diseases. In the present study, we analyzed the molecular forms of plasma BNP in ESRD patients of both before (pre-) and after (post-) hemodialysis (HD) state. The plasma extract was analyzed by gel filtration on a TSK-GEL G2000 SW column followed by a RIA for both BNP and atrial natriuretic peptide (ANP). In the pre-HD patients, a 14- to 2300-fold increase in plasma level of immunoreactive (ir-) BNP was observed when compared to normal controls. A ratio of BNP-32 to g-BNP (pro BNP) in plasma from the patients was much larger than that in plasma from normal subjects, indicating that the high plasma level of ir-BNP level in the patients on HD largely results from a marked increase in BNP-32. HD significantly (P < 0.01) lowered the plasma levels of both BNP-32 and g-BNP with a greater reduction in BNP-32 than in g-BNP. Whereas, a-ANP was a main molecular form of plasma ANP in both pre- and post-HD plasma. These results suggest that plasma BNP-32 plays an important role in the sodium-fluid balance and that secretion and metabolism of BNP may differ from those of ANP in the HD patients.