The rate of loss of NH3 from water to the atmosphere was measured in the field and in the laboratory as a function of wind speed, temperature and pH. The exchange coefficient, Ke, was a linear function of wind speed and temperature. The loss rates (1-10 kg .cntdot. ha-1 .cntdot. day-1) are quite high at pH 9 and at NH3 concentrations > 1 mg N .cntdot. dm-3; the rates are much smaller under the conditions found in most natural water bodies.