The decrease in soil nitrate nitrogen as related to redox potential was studied in the laboratory and in the field in Imperial Valley, Calif.Partial anaerobic conditions were induced in laboratory flasks containing an atmosphere of 21% oxygen by adjusting the soil‐water ratio. When the water content of a soil having a water saturation percentage of 48% was increased to 44.5% or above, large losses of N2 gas occurred both with and without addition of organic matter. Significant denitrification coincided with a decrease in the redox potential (Eh).Measurement of redox potential and nitrate concentrations of ground water sampled at various depths in an irrigated field indicated that nitrate concentration was high near the surface but decreased at depths approaching the water table. The diminishing nitrate concentration and redox potential, with depth, indicate that some nitrates were reduced to nitrogen gas before they reached the water table.Only 1.5% of the 280 kg N/hectare applied to a cotton crop (Gossypium hirsutum L.) was discharged in the tile effluent during the growing season. Denitrification in the soil profile apparently reduced the amount of nitrate reaching the tile drainage system.