OH concentrations in ambient air outside Ford Laboratory were measured in the months of July and August 1986 using the fluorescence technique at low pressures. The peak OH concentration was observed to be about 3¿106 molecules/cm3; the daytime average OH concentration was found to be in the range of middle 105 molecules/cm3, and the nighttime average below the detection limit of 0.9¿105 molecules/cm3. It is demonstrated that use of moving average may create the appearance of oscillatory behavior in the diurnal variations of the deduced OH concentration. ¿ American Geophysical Union 1987