Interest in the olfactory mucosa has increased in recent years, since it has been shown to possess a considerable amount of cytochrome P-450-dependent monooxygenase activity and a wide variety of chemicals have been identified as olfactory toxins. Many chemicals induce lesions of a general nature in the olfactory mucosa, i.e., inflammation, degeneration, regeneration, and proliferation, whereas others cause more specific effects. Changes in the olfactory mucosa with reference to chemicals that initiate them are reviewed in this paper. Studies with 3-trifluoromethyl pyridine (3FMP) illustrate some of these general changes and show the importance of examining the olfactory mucosa at early time periods. The earliest damage seen by light microscopy was 6 hr after a single inhalation exposure to 3FMP, and by day 3, early regenerative changes were observed. Changes were seen by electron microscopy 30 min after an oral dose, and the primary site of toxicity appeared to be the Bowman's glands. Although atrophy of nerve bundles in the lamina propria would be the expected consequence of severe necrosis of the sensory cells, this is not always the case. Exposure to irritants such as acetaldehyde, formaldehyde, and dimethylamine results in nerve bundle atrophy, but with chemicals such as 3FMP, 3-methylindole, and 3-methylfuran--which are activated by mixed-function oxidases--the nerve bundles remain intact. Future work, including metabolism studies, will provide information on the mode of action of these chemicals.