The distribution of the plasma levels of atrial natriuretic peptide (ANP) and their statistical correlates were studied in a random sample of 306 untreated male workers in southern Italy. Plasma ANP was not normally distributed in the population sample, with values skewed to the right and significantly shifted from the normal Gaussian curve (p = 0.003). In a multivariate analysis, plasma ANP was independently and positively related to age (p less than 0.001), urinary sodium excretion (p less than 0.01), and inversely related to creatinine clearance (p less than 0.02).These variables explained more than 12% of the variability of plasma levels of ANP. These findings confirm, in a random sample of untreated male workers, the importance of age and renal function in predicting ANP levels in the plasma and emphasize the potential importance of ANP in the control of sodium excretion in humans under free living conditions.