Spectroscopic techniques have shown that quaternary benzophenanthridine alkaloids (sanguinarine (Ia), chelerythrine (Ie), sanguilutine (If), sanguirubine (Ic), chelirubine (Ib), and chelilutine (Id)) are reduced by NADH or NADPH to their dihydro forms. The formation of the oxidized form of a coenzyme was demonstrated by HPLC. The kinetics of the reaction of sanguinarine with NADH has been studied rather in detail; the reaction was found to be reversible, its stoichiometry was 1 : 1. Reactivity of the tested alkaloids toward the coenzymes was given by reactivity of the imine bond.This was graded like pK values for the formation of the corresponding pseudobases and correlated with the energy of the lowest unoccupied molecular orbital of these compounds. The possible biological and pharmacological importance of the studied reactions is discussed.