In an incubation experiment, both biological and chemical reduction of nitrate were studied under flooded conditions at different redox (0, 200 and 400 mV) and pH (5, 7 and 9) values in the presence and absence of HgCl₂ as a sterilant.                 Nitrate reduction followed a linear pattern with the time of incubation at all   combined pH – Eh treatments. At a given pH value, NO^-₃ reduction increased by 2 to     3 folds due to lowering Eh from 400 to 0 mV. The highest reduction rate was found at pH 9 combined with Eh 0 mV. At 48 hours of incubation, for instance, and pH 9 the reduction was 73% at 0 mV but didn't exceed 24% at Eh 400 mV in the absence of HgCl₂. The corresponding values in the presence of HgCl₂ were 80% and 42%, respectively. Catalytic action of HgCl₂ under alkaline conditions was responsible for  such increase by enhancing the chemical reduction. Under acid conditions, at 0           and 400 mV without HgCl₂ the values were 64% and 30%, but were 25% and 14% with HgCl­₂. Such decreased values were attributed to the poisoning action of soluble   mercury under acid conditions to soil biota. It also indicated the importance of    chemical reduction under acid and well aerated conditions which are different than    those suitable for nitrate reducing microorganisms.