The purpose of this study was to evaluate the role of C fibers in airway responsiveness after exposure to ozone (O3) in rats. The role of C fibers in the decreases in heart rate (HR) and core body temperature (Tc) that occur after inhalation of O3 was also examined. Neonatal rats were treated with capsaicin (Cap) or the vehicle used to dissolve capsaicin (Veh). Cap has been shown to cause permanent destruction of C fibers. When they reached adulthood, conscious minimally restrained rats were exposed to 2 ppm O3 or to air for 3 h. Two hours after the cessation of exposure, rats were anesthetized and instrumented for the measurement of pulmonary mechanics and airway responsiveness to inhaled aerosolized methacholine. O3 had no effect on baseline pulmonary conductance (GL) in either Veh or Cap rats but did cause a decrease in dynamic compliance (Cdyn) in Cap rats (P < 0.05). In Cap rats, O3 exposure caused a marked increase in airway responsiveness; the doses of inhaled aerosolized methacholine required to decrease GL and Cdyn by 50% were 6.5-fold and 9.8-fold lower in O3-compared with air-exposed rats (P < 0.005). In contrast, in Veh rats, O3 did not alter responsiveness. During O3 exposure, there was a profound, almost 50%, decrease in HR as measured with implanted electrodes. A decrease in Tc (measured with a rectal probe) of approximately 2.5 degrees C also occurred during O3 exposure. There was no significant effect of Cap pretreatment on the magnitude of these O3-induced changes in HR and Tc. Our results are consistent with the hypothesis that C fibers act to inhibit the development of hyperresponsiveness elicited by O3 inhalation but do not contribute to O3-induced changes in HR or Tc.