In nitrate reductase preparations from radish cotyledons an endogenous inhibitor of a non competitive nature was found. A method for calculating the amount of inhibitor present was devised. The amount of inhibitor present in one pair of cotyledons decreased with increasing age of the seedlings, in light as well as in darkness. It is suggested that the inhibitor is of a phenolic nature since the inhibition could be reversed by extraction with polyvinylpyrrolidone.