A miniaturized Flow Injection system for the assay of urea in undiluted whole blood is described. Based on the optical determination of ammonia, the system incorporates a flow cell combining gas diffusion and optosensing, the separating barrier between the donor and accepting streams consisting of a sandwich of a hydrophobic gas permeable membrane and a hydrophilic membrane between which is contained a gel of covalently immobilized urease. The sample urea content is quantified by the colour change of an acid-base indicator contained within the acceptor stream. Within the physiological range the measurement is not affected by variations in the pH value, the buffering capacity or the hematocrit level of the sample solution, nor is it, due to the geometric construction of the flow cell, prone to interferences owing to the colour of the sample solution itself. The system is stable for a t least one week of continuous use.