Nutrient discharge from a paddy field cultivated with barley was investigated. It was shown that 1) T-N concentration gradually rose to the maximum value (approximately 40 mg/l concentration) about 60-70 days after the fertilization and fell rapidly to a low level (1-3 mg/l), 2) 99-100% of T-N in drainage water was in the NOX-N form and 3) 96-100% of the N was discharged through the underdrain. Based on these data, a mathematical model to estimate the N discharge from the field was developed. The unique features of this model are as follows: 1) nitrogen in a field occurs in 6 forms; stable organic N, unstable organic N, exchangeable ammonium N, soluble ammonium N, nitrate/nitrite N and urea N, 2) various transformation processes; nitrification, denitrification, mineralization, demineralization, urea hydrolysis and ion exchange were assumed to follow a first-order kinetics, 3) Arrhenius law was applied to the temperature dependence of transformation rates, and 4) N concentration of the field drainage is assumed to be directly proportional to the nitrate/nitrite nitrogen content in the field. It is possible to trace the observed data of N discharge from the underdrain during 2 periods of barley cultivation in the paddy field. This model can contribute to the prediction of N discharge from a field in conjunction with a field drainage model.