Experimental measurements of ozone (O3) uptake are needed for validation of dosimetry model parameters and in predictions as well as for determining factors affecting uptake and for making comparisons between subpopulations or across species. In this study, 10 healthy adult male subjects were exposed to 0.3 ppm O3 while seated and breathing naturally through the nose or mouth. Total respiratory tract O3 uptake, spontaneous breathing parameters, and respiratory gas exchange were measured for 10 min under steady-state conditions. The exposure protocol was replicated in each subject approximately 2 weeks after the first visit. On each visit, health exams were performed and spirometric lung measurements were obtained. The experimental design provided comparisons of total O3 uptake during nasal and oral breathing, differences in uptake in an individual at two time points, and an examination of between-subject variability in O3 uptake. Exposure to O3 had no effect on the breathing parameters or gas exchange. Oral and nasal breathing frequency averaged 16.2 +/- 1.1 (SE) and 16.0 +/- 1.2 breaths per minute with tidal volumes averaging 651 +/- 46 and 669 +/- 67 ml, respectively. A significant correlation (p < 0.01) was found for the minute volume during resting breathing with the percentage of uptake. The percentage of O3 uptake was consistently higher (p = 0.02) during oral breathing (76.5% +/- 3.3) than during nasal breathing (73.1% +/- 3.0) although this difference may not be biologically significant. The variability in percentage of uptake between subjects was substantial with calculated uptakes ranging from 51 to 96%, a difference of about 45%. Variability in percentage of uptake for an individual was less with the maximal difference between the first and second visits being about 20%; the average difference, however, was only about 3%. We conclude that total percentage of O3 uptake is approximately 75% in adult males during resting breathing. It is slightly greater during oral than during nasal breathing, will vary considerably among subjects, and is moderately reproducible within a subject.