Abstract: An isothermal distilaltion unit is incorporated in flow injection systems. The influence of surfactant, flow rates, alkalinity, ionic strength, collector stream pH, reagent concentration, and sample volume in NH3 distillation are discussed. A method for the determination of total N in plant digests employing Nessler reagent is proposed. This method, based on merging zones approach, is characterized by a sampling rate of 100 samples/h, a precision >3%, a reagent consumption of only 100 µL/sample, and virtually no base-line drift. The results agree with those obtained with the indophenol blue colorimetric method. The incorporation of isothermal distillation in flow systems with potentiometric measurement is also described.