A method is presented to evaluate pulmonary performance in unrestrained and unanesthetized guinea pigs by measuring the increase in tidal volume (VT), respiratory frequency (f), and minute ventilation (VT · f) induced by exposure to a mixture containing 10% CO₂, 20% O₂, and 70% N₂. The increase was measured before and after exposure to various concentrations of sulfuric acid mist. Exposures to sulfuric acid produced a concentration-dependent reduction in CO₂-induced increases in VT, f, and VT · f. Recovery from pulmonary injury induced by exposures to sulfuric acid was apparent from daily CO₂ challenge. The method is simple, noninvasive, and required neither physical restraints nor anesthesia. The results of the CO₂ challenge were reproducible for each animal over a 5-day period.