The relationship between segmental renal tubular sodium handling (using the renal clearance of ingested lithium as a marker of proximal tubular sodium handling) and circulating plasma levels of atrial natriuretic factor (ANF) was studied in a sample of 295 untreated men drawn from a male population at work under their usual living conditions. Plasma ANF was positively and significantly related to sodium excretion at the distal nephron, indicating that this hormone interacts with a distal renal tubular site to influence the control of sodium excretion in man.