Cellular activity of nitrate reductase in Pseudomonas denitrificans which had been grown under denitrifying conditions was increased several times upon incubation of cell suspension with monovalent cations. The enhancement of nitrate reductase activity caused by monovalent cations was ascribed to the activation of the enzyme, since the membrane fraction isolated from the cells after the cation treatment retained the elevated levels of enzyme activity. However, monovalent cations had no effect when added directly to cell-free homogenate, suggesting an important role of some definite structure of membrane in the expression of the effect of monovalent cations.