Plasma concentrations of atrial natriuretic peptide (ANP) were measured in nine infants (age 4 days-9 months) before and after closure of patent ductus arteriosus. Initially all patients had marked distention of the left atrium as indicated by a left atrium to aortic root ratio greater than or equal to 1.3 on echocardiography. After closure of the ductus, operative in six and pharmacological in three patients, left atrial size normalized (left atrium to aortic root ratio less than 1.3) in all patients, except in one treated surgically. Before closure the plasma concentration of ANP was 86-2224 pg/ml and after closure 44-1400 pg/ml. There was a significant correlation between the size of left atrium and plasma concentration of ANP (r = 0.56; p = 0.01). Our results suggest that in infants with patent ductus arteriosus the left atrium is the main source of ANP. The secretory rate of ANP decreases immediately after restoring left atrial size by closure of the ductus.