The activation process of pepsinogen was followed by the appearance of peptic activity and the conformational changes measured by difference spectrophotometry. The activation process of pepsinogen followed by peptic activity exhibited characteristic features ; for instance, it showed a clear induction period at low temperature and no sensitivity for a competitive inhibitor isoamyl alcohol. The conformational changes accompanied by the activation to appear behind the appearance of the peptic activity and showed several distinguishable stages. Furthermore, it has been well known that the activation of pepsinogen took place by acidification of the pepsinogen solution without any assistance of other proteolytic enzyme and that the activation mechanism depended upon the pH value of medium used for the activation. Thus, the activation mechanism of pepsinogen has been considered to be of very complex. In the present experiment, the activation process was simulated by analog computer with postulating several schemes in order to classify the plausible schemes which can explain the observed characteristic features in Ihe activation process of pepsinogen. Then, kinetic parameters in most probable schemes were determined by digital computer, using a nonlinear programming technique.