We have developed a radioimmunoassay for the measurement of immunoreactive BNP (1-32) in human plasma. Simultaneous measurement of ANP have also been carried out to allow for direct comparison between circulating BNP and ANP. Plasma levels of immunoreactive BNP (means +/- SEM) were 1.1 +/- 0.1 pmol/l in 36 normal healthy subjects and were significantly elevated in 50 patients with essential hypertension (1.6 +/- 0.2 pmol/l, P < 0.02). Similarly, in patients with essential hypertension plasma levels of ANP were also significantly raised (5.5 +/- 0.6 pmol/l, P < 0.001) when compared with the group of normal healthy subjects (2.8 +/- 0.2 pmol/l). ANP was significantly higher than BNP in normal subjects and in patients with essential hypertension, with ANP/BNP ratios of 2.8 +/- 0.2 and 3.8 +/- 0.3, respectively, in these two groups. A major finding was a significant and positive association between plasma levels of both BNP and ANP within the healthy subjects (r = 0.49, P < 0.05, n = 36) and within the hypertensive subjects (r = 0.76, P < 0.001, n = 50). When all plasma values for BNP and ANP were taken together for both groups, there was an overall correlation coefficient of 0.65 (P < 0.001, n = 86). Both BNP and ANP had significant positive associations with age in hypertensive patients, with correlation coefficients of 0.53 (P < 0.001, n = 50) and of 0.53 (P < 0.001, n = 50) for BNP and ANP, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)