The NASA Goddard Space Flight Center two-dimensional (2-D) model has been used to study the sensitivity of model ozone concentrations to input chemical reaction rates, and the uncertainty of the model-calculated concentrations. Ozone sensitivity coefficients to changes in chemical reaction rates are defined as logarithmic partial derivatives of the ozone concentration with respect to the chemical reaction rates. These logarithmic derivatives are estimated using a finite difference technique. The ozone sensitivity coefficients to 96 gas phase chemical reactions in the 2-D model show that the ozone concentration is sensitive to the rates of about 25 reactions. The magnitude of the ozone sensitivity coefficients varies from 0.05 to 0.9. The latitude-altitude distributions of the ozone sensitivity coefficients to several reactions are presented. The uncertainty of the model-calculated ozone concentration is evaluated using a guided Monte Carlo (GMC) method from a probability distribution function. The GMC method judiciously combines uncertainty estimates derived from the sensitivity information with Monte Carlo runs of the model. The uncertainty of the model ozone concentration due to uncertainties in gas phase reaction rates is calculated from published chemical rate uncertainties and varies from 10--20% in the lower stratosphere to 30--40% in the mesosphere. Details concerning the GMC method are discussed, and the latitude-altitude distribution of the uncertainty of the model-calculated ozone is presented.¿ 1997 American Geophysical Union