The concentration and molecular form of the plasma atrial natriuretic peptide (ANP) in normal calves during the first 10 days of life were investigated. The mean (SEM) ANP concentration on the day of birth was 62.7 (5.52) pmol litre-1, and thereafter it decreased progressively and significantly (P < 0.001). After 10 days, the plasma ANP concentration had decreased to normal adult values (11.3 [3.87] pmol litre-1). On the day of birth and 10 days later a single peak corresponding to alpha-ANP alone was detected in the plasma of the calves.