The atrial natriuretic factor is a peptide with vasodilator and natriuretic properties which is synthesized and released by atrial myocytes. In patients with chronic heart failure the ventricles contribute to the synthesis of this peptide, so that its plasma concentration rises in proportion to the heart filling pressure. The atrial natriuretic factor, therefore, seems to be a good biological marker of functional and haemodynamic severity of heart failure. However, the weight of this factor in the neuro-hormonal balance has not yet been fully evaluated, owing to the usual attenuation of its biological effects. This, and the necessity of continuous parenteral administration limits, for the moment, its therapeutic applications. Inhibitors of the enzyme involved in its metabolism are being tested, but only preliminary results are available in humans.