Nitric oxide has been measured in situ between 38.4 and 29.8 km by a new technique, photoionization mass spectrometry. Data indicate a peak mixing ratio of 5.7 ± 0.35 ppbv at 37.5 km and only 0.2 ± 0.2 ppbv at 32.8 km with a sharp gradient in concentration between 33 and 34 km. Simultaneous in‐situ measurements of the ozone concentration show no significant feature at that altitude. The rapid decrease in mixing ratio is not predicted by current the