The reflex and resorptive effects of crotonaldehyde was studied. The maximum acceptable concentration (MAC) of crotonaldehyde was established to be 0.025 mg/m3, the mean daily MAC recommended to be 0.003 mg/m3, Hazard Class 2. It was shown that nasopharyngeal mucosal morphological changes in laboratory animals might serve as significant criteria for the action of the agent having an irritant effect.