The natriuretic peptide system consists of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). The system is implicated in the control of body fluid homeostasis, causes natriuresis and diuresis (ANP and BNP), and regulates vascular tone (CNP). A reciprocal relationship between ANP and endothelin (ET) has been suggested, and earlier studies have documented a possible role of ET in cerebral vasospasm after aneurysmal subarachnoid hemorrhage (SAH). The authors studied plasma ANP, BNP, CNP, and ET for 6 consecutive days in 13 patients with SAH by using radioimmunoassay. The median admission values for ANP were 31.5 pg/ml (range 16.8-323 pg/ml [normal 15 +/- 7 pg/ml]); for BNP, 45.3 pg/ml (range 2.2-80.2 pg/ml [normal 12 +/- 9 pg/ml]); for CNP, 7.7 pg/ml (range < 2-20 pg/ml [normal 5.2 +/- 3 pg/ml]); and for ET, 11 pg/ml (range 6.5-25.1 pg/ml [normal 7.2 +/- 4 pg/ml]). Additional increases (defined as > 100% increase on two consecutive measurements) were noted in ANP (11 patients), BNP (10 patients), and CNP (three patients), and resulted in a negative fluid balance in 10 of the 13 patients. The CNP increased in three of four patients with cerebral vasospasm and in one of nine patients without cerebral vasospasm (Fisher's exact test, p = 0.2). No major fluctuations in plasma ET were noted. In seven patients, the plasma ET level did not increase beyond 10 pg/ml during the days of measurement. In six patients, only an occasional sample showed an increase to a maximum of 25 pg/ml. Changes in BNP, ANP, and CNP were independent of each other. The authors conclude that both plasma ANP and BNP increase after SAH and often result in a negative fluid balance. Plasma ANP and BNP seem differentially regulated in the presence of SAH but not by the level of the plasma ET. The possible role of CNP as a regulatory response to cerebral vasospasm needs further exploration.