AbstractThe syntheses of seven tripeptide isomers containingL‐histidine,L‐proline andL‐glutamic acid residues, the same as found in the natural thyrotropin‐releasing hormone (TRH), are reported. In additionL‐pyroglutamyl‐L‐histidyl‐L‐proline and its amide as well as Nα‐acetyl‐L‐glutamyl‐L‐histidyl‐L‐proline are described. Whereas eight peptides are inactive andL‐pyroglutamyl‐L‐histidyl‐L‐proline shows a slight TRH activity,L‐pyroglutamyl‐L‐histidyl‐L‐proline‐amide has the full biological activity of the isolated thyrotropin‐releasing hormone and, at the p