Studies are reported on the denaturation of freshly prepared, intact swine pepsin, which was inactivated by reaction with diazoacetylglycine ethyl ester, to prevent autolysis. Denaturation about pH 6 was found to involve a small expansion of the molecular domain with some loss of organized secondary structure. On the other hand, increasing concentrations of guanidine hydrochloride induced cooperative transitions in both the native and alkali denatured forms to give a cross-linked random coil. No conditions could be found in which these reactions were reversible. Removal of denaturing conditions usually resulted in aggregation and precipitation of protein. From these studies, it would seem that the active conformation is largely predetermined in the zymogen.