The effect of inhaled irritant gas, O 3, on thyroid function of rats has been studied. The following results emerged: (1) Single 5-hour exposures of O 3 in concentrations of 4, 2 and 1 ppm produced inhibition of thyroid release of I 131, an effect which was significant for a period of from 48 to 384 hours. (2) Repeated 5-hour exposures to O 3 in either 0.5 or 1.0 ppm concentration on each of 4 days, did not significantly alter thyroid I 131 content when measured from 96 to 384 hours. These exposures are known to produce a high degree of tolerance to subsequent challenge. (3) Repeated 5-hour exposures to O 3 in 2 ppm concentrations on each of 2 days, followed 3 days later with a challenge exposure of 4 ppm for 5 hours, not only failed to inhibit thyroid activity, but actually increased the release rate of I 131. Results are discussed relative to a tri-phase response characterized primarily by a depression of thyroid function followed by a period of resistance wherein function appears normal, terminating in a phase of thyroid overactivity. These responses to O 3 inhalation indicate that the tolerance phenomenon induced by certain irritant gases is associated with, but not necessarily determined by, a resistant phase of thyroid activity, and furnishes evidence for activation of humorally mediated mechanisms by this gas in concentration as low as 0.5 ppm.