The structural stability of salivary gland hormone, parotin was investigated by circular dichroism (CD) measurement. The protein was denatured between pH 4 and 5, but it was stable at other pH regions. The relaxation time of denaturation was few minutes and the pH denaturation was readily regenerated by neutralization. This protein was particularly unstable to heat; it began to denature even at a low temperature and the reproduction from thermal denaturation was difficult. In the model experiment where parotin was presumed as receptor protein, several neurotransmitters altered the conformation of it.