Significant diurnal variations in the concentration of nitrous oxide (N2O) in surface air were detected by analysis of air samples collected at two sites in Iowa. At both sites, the highest concentrations of N2O were observed during early morning hours. The diurnal variations observed decreased with increasing height above ground level and indicated that soil in the areas studied was a source of atmospheric N2O. They differed markedly from the variations detected in recent work leading to postulation of the existence of a major ground surface sink for atmospheric N2O.