Extensive in situ measurements of reactive nitrogen (NOy) and ozone (O3) were made in the lower stratosphere over a broad latitude range (60¿N--70¿S) during two different seasons (March and October) in 1994. Both NOy and O3 mixing ratios show a strong latitude dependence, with values increasing toward the poles. The NOy/O3 ratio reveals a high-gradient region near the tropics that is not well-represented in standard 2-D photochemical transport models. Improving the representation by changing the horizontal eddy-diffusion coefficients near the tropics has important implications for the predicted impacts of aircraft emissions on stratospheric O3. ¿ American Geophysical Union 1996