Abstract Secretin has repeatedly been said to be unstable. We found that the biological potency of secretin solutions under some conditions decreased. We tested hypotheses about the cause(s). Adsorption of secretin on materials in which it is stored may account for most of the rapid decrease of biological potency. Some intramolecular reactions - Asp( α→ β)-rearrangements and peptide chain cleavage - also occur. These reactions are possible causes of a slow decrease of biological potency.