Studies were conducted to assess the effects of bolus injections and infusions of human atrial natriuretic factor (ANF) in control subjects and patients with mild essential hypertension, and to measure plasma immunoreactive ANF (irANF) concentration in a large group of patients with essential hypertension. The results are compared with those obtained by other groups on the measurements of plasma irANF in hypertensive patients. It appears that plasma irANF concentrations are not increased in patients with mild essential hypertension despite the evidence of increased preload and of atrial distention as reported by others. This suggests a hyporesponsiveness of the atria to release ANF.