The altitude distribution of the nitric oxide nightglow has been measured with the ultraviolet spectrometer on board Pioneer Venus. Limb profiles obtained near periapsis with a ~5 km vertical resolution have been fitted using a method independent of the spacecraft attitude data. It is found that the altitude of the maximum of the layer is 115¿2 km and the topside scale height ?3 km. It is shown that downward transport by diffusion alone is insufficient. If vertical motion is parametrized by eddi diffusion an eddy diffusion coefficient K≈8+8-4¿1012/{M}1/2 cm2. s-1 is deduced from the altitude of the layer.