Above pH 8.5, pepsinogen is converted into a form which cannot be activated to pepsin on exposure to low pH. Intermediate exposure to neutral pH, however, returns the protein to a form which can be activated. Evidence is presented for a reversible, small conformational change in the molecule, distinct from the unfolding of the protein. At the same time, the molecule is converted to a form of limited solubility, which is precipitated at low pH, where activation is normally seen. The results are interpreted in terms of the peculiar structure of the pepsinogen molecule. Titration of the basic NH2-terminal region produced an open form, which can return to the native form at neutral pH, but which is maintained at low pH by neutralization of carboxylate groups in the pepsin portion.