To detect ammonia using an electrochemical method, we developed a sensor chip consisting of L-glutamate dehydrogenase, L-glutamate oxidase, and a Prussian blue-carbon electrode. When the enzyme electrode was applied to potentiometric and amperometric ammonia measurements, the limits of detection were 0.2 mu M and 2,100 mu M, respectively. Thus, potentiometric measurements, in which the charge produced by the enzymatic reaction accumulates on the electrode, allow more sensitive detection of ammonia than amperometric measurements, in which the charge passes through the electrode.