The sensory irritant properties of ozone have been considered to be responsible for symptoms that occur in humans after exposure. This assumption has not been studied explicitly. One way to assess the aversive properties of airborne irritants is to give the exposed individual an opportunity to control the duration of exposure, i.e., escape from the irritant. Mice were trained to turn off 100-ppm ammonia, a concentration that, in humans, is irritating to the upper airways. After the determination of ammonia concentration-effect curves, ozone was substituted for ammonia. Additional mice with no history of ammonia termination were trained to terminate ozone exposure, indicating that the aversive properties of ozone were not dependent on previous experience with other airborne irritants.