A rapid and highly sensitive method is described for reliable flow injection analysis of urea based on pulsed-amperometric detection with a polypyrrole-urease biosensor. The optimum conditions were 0.05 M phosphate buffer (pH 7.0), a negative potential pulse between -70 mV (E 1) and -400 mV (E 2) VS Ag/AgCl, a pulse width of 120 mS, and a flow rate of 0.1 mi min -1. A detection limit (3σ) of 60 μg l -1 of urea was achieved by this method and the response was linear in the range 100-4500 μg l -1. The biosensor was very stable in the flow system, even after two weeks of continuous use. Excellent recovery of urea (95%-112%) was accomplished employing this method in simulated samples. Successful application of the method to the determination of urea in blood plasma is reported.