The efficiency of ozone removal from inspired air by the extrathoracic and intrathoracic airways was measured in 18 healthy, nonsmoking, young male volunteers. Removal efficiencies were measured as a function of ozone concentration (0.1, 0.2, and 0.4 ppm), mode of breathing (nose only, mouth only, and oronasal), and respiration frequency (12 and 24 bpm). Subjects were placed in a controlled environmental chamber into which ozone was introduced. A small polyethyethylene tube was then inserted into the nose of each subject with the tip positioned in the posterior pharynx. Samples of air were collected from the posterior pharynx through the tube and into a rapidly responding ozone analyzer yielding inspiratory and expiratory ozone concentrations in the posterior pharynx. The mean extrathoracic removal efficiency for all measurements was 39.6%, and the mean intrathoracic removal efficiency was 91.0%. Significantly less ozone was removed both extrathoracically and intrathoracically when subjects breathed at 24 bpm compared to 12 bpm. Ozone concentration had no effect on extrathoracic removal efficiency, but there was a significantly greater intrathoracic removal efficiency at 0.4 ppm than at 0.1 ppm.