A simulation of the altitude distributions of HNO3, NO2, and NO measured on the Stratoprobe balloon flight of August 28, 1976, has been carried out by using a time-dependent stratospheric model with extensive photochemistry. The measured ozone and temperature profiles were employed in the simulation. The simulated ratios for HNO3/NO2 and HNO3/NO are significantly larger than the measured ratios for HNO3/NO2 and HNO3/NO in the altitude range 20--30 km. Since these ratios are proportional to the hydroxyl densities, it is proposed that the actual hydroxyl densities are a factor of 3--5 lower than calculated in the model, using current chemical rate data. By assigning a pressure dependence to the OH+HO2 reaction rate as well as including the OH+HNO3 reaction rate to reduce the OH densities in the model over this altitude range, an empirical simulation of the measured ratios was obtained. The pressure dependence chosen for the rate was such that reasonable agreement with existing mesurements of hydroxyl densities at higher altitudes was still obtained.