Isolation and sequencing of IR-ANF from plasma reveal that the circulating form of ANF is a 28-amino acid peptide (Ser 99-Tyr 126) which inhibits basal aldosterone secretion from suspensions of rat zona glomerulosa cells to the same extent as synthetic Arg 101-Tyr 126. The same form (minus two N-terminal amino acids) (Arg 101-Tyr 126) has been isolated from culture media of atrial cardiocytes and sequenced. Structure activity studies of shorter, N- or C-terminally truncated versions of synthetic Arg 101-Tyr 126 show that the latter is the most potent one.