A highly sensitive and rapid flow injection system for urea analysis was constructed with a composite film of electropolymerized inactive polypyrrole (PPy) and a polyion complex incorporating urease. This system shows a sensitivity of 120 mV decade(-1) and a lifetime of more than 80 assays. The origin of the high sensitivity of this system is attributed to an additional potential response of inactive PPy to ammonia or ammonium ion superimposed on the response to pH change. By injecting a concentrated buffer solution immediately after the sample injection, this system is capable of assaying more than 15 samples per hour.