Photographic properties of 1-benzyl-1,4-dihydronicotinamide (BNAH) are studied for better understanding of the mechanism of the production of high contrast by the infectious development which is induced by the combination of a pyridinium salt and Metol-ascorbate developer. Treatment of internally chemically sensitized silver bromide core/shell emulsion by BNAH and redox buffer followed by development gave high internal fog density. It has been suggested in that the oxidation product of BNAH by redox buffer injects electrons into the conduction band of silver bromide grains, resulting in the formation of fog centers inside the grains. Results support the hypothesis that dihydropyridine works as an intermediate in the production of high contrast by the infectious development in the presence of a pyridinium salt.