On the basolateral infoldings of the strial marginal cells in the cochlea, Na K ATPase activity is abundant. To clarify the humoral control by norepinephrine, K-NPPase activity of strial marginal cells in the cochlea was investigated in normal, reserpine, norepinephrine (NE), reserpine plus NE-treated guinea pigs using a cerium-based method. K-NPPase activity was almost completely decreased 3-20 days after reserpine administration. At 10 days after reserpinization and following NE repeated treatment, enzyme activity was detectable. These results suggested that norepinephrine might restore and regulate strial K-NPPase activity.