1. Eight cyclo-alkyl lactamimides have been investigated for potential inhibitory action upon the pepsins and pepsinogens.
2. Human pepsins 1, 3 and 5 and swine pepsin were inhibited only slightly.
3. Human and swine pepsinogens were inactivated progressively by lactamimides as the number of methylene groups in the nitrogen-containing ring increased. The most potent in activator studied was N-(cis-2-phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride.
4. Substitution of benzyl and tertiary butyl groups in the N-containing ring increased the pepsinogen-inactivating property of the cyclo-alkyl lactamimides.
5. N-(cis-2-Phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride may be of potential importance as a therapeutic agent in peptic ulcer, and modifications to the molecule which might increase its pepsinogen-inactivating ability are suggested.