A heptacosapeptide having the following structural formula: Boc-His(Boc)-Ser(Bu.sup.t)-Asp(OBu.sup.t)-Gly-Thr(Bu.sup.t)-Phe-Thr(Bu.sup. t)-Ser(Bu.sup.t)-Glu(OBu.sup.t)-Leu-Ser(Bu.sup.t)-Arg(X)-Leu-Arg(X)-Asp(OBu .sup.t)-Ser(Bu.sup.t)-Ala-Arg(X)-Leu-Gln-Arg(X)-Leu-Leu-Gln-Gly-Leu-Val-NH. sub.2, in which X is a group having the formula: ##STR1## where R is an alkyl or alkoxy and m is an integer of 1 to 3, provided that R's may be the same or different in case m is 2 or 3. The heptacosapeptide may be converted to a secretin.