An analytical system specially built for on-line urea monitoring is reported. Measurements are carried out in the effluent of a haemodialysis machine. The measuring system employs the dialyser inflow stream as a carrier solution channel in a continuous fashion. The analyser periodically samples the outflow stream of the dialyser by means of an automatic injection valve. The analyser features a bioreactor consisting of immobilized urease and a gas-diffusion module. It is through this module that the urea is converted to ammonia gas which is transferred to another carrier channel, this transports the ammonium ion to a tubular, all-solid-state, ion-sensitive electrode. A timer controls the transport, injection, the measuring and the recording subsystems. The analyser has been used during actual haemodialysis sessions. Urea clearances were also measured in batch, using conventional spectrophotometric clinical equipment. The correlation between both methodologies was sufficient to confirm the usefulness of the developed on-line analyser to monitor the optimal length of haemodialysis sessions.