Recently several peptides with natriuretic and diuretic potencies were isolated from human and rat atrial extract, and the precursors of the peptides were sequenced. Of the peptides, α-human and rat atrial natriuretic polypeptides (α-hANP, α-rANP), consisting of 28 amino acids, are thought to be essential to the potency and to play an important role in the blood pressure regulation system. The amino acid sequence of α-hANP $${\text{(SLRRS}}{\text{NSFRY)}}$$ is different from that of α-rANP only at the position 12 (isoleucine in α-rANP). In the present study, we synthesized ANPs and their analogs using a new deprotection procedure based on the concept of push-pull mechanism. Using the synthetic ANP analog, we also developed a radioimmunoassay for α-ANP and examined the structure-activity relationship. Synthetic α-hANP caused potent, rapid, and short-acting increases in Na+ and Cl− excretion, and also an increase in urine flow and K+ excretion of lesser magnitude, when injected into rat. Also, we synthesized a cyclic part of α-hANP, α-ANP(7–23)-NH2. Since this peptide had a little diuretic and natriuretic potency, we attempted to synthesize a chemically stable α-hANP analog. We considered that the disulfide bond would be equivalent to propylene with regard to interatomic distance and employed 8-aminocaprylic acid instead of cystine. This cyclic peptide, named cyclonatrin-54, had a somewhat higher potency than α-hANP(7–23)-NH2 for diuresis and natriuresis, as expected. Furthermore, using a synthetic intermediate of cyclonatrin-54, we prepared a linear ANP analog, α-hANP(8–22), Phe-Gly-Gly-Arg-Met-Asp-Arg-Ile-Gly-Ala-Gln-Ser-Gly-Leu-Gly. This linear 15-amino acid peptide had a dose-dependent natriuretic and diuretic activity, but no hypotensive effect. It was surprising that a linear peptide exhibited a potent natriuretic activity. For the first time, a linear peptide has been prepared that has substantial natriuretic and diuretic potency. We synthesized some analogs of this 15-amino acid peptide and investigated the structure-activity relationship.