Dimethylamine is a widely used commodity chemical, for which there are few chronic toxicity data. Male and female F-344 rats and B6C3F1 mice were exposed by inhalation to 0, 10, 50, or 175 ppm dimethylamine (DMA) for 6 hr/day, 5 days/week for 12 months. Groups of 9-10 male and female rats and mice were necropsied after 6 and 12 months of exposure. No male mice were sacrificed at 12 months due to a high incidence of early deaths in that group. The mean body weight gain of rats and mice exposed to 175 ppm DMA was depressed to approximately 90% of control after 3 weeks of exposure. The only other treatment-related changes were concentration-related lesions in the nasal passages. Two distinct locations in the nose were affected: the respiratory epithelium in the anterior nasal passages, and the olfactory epithelium, especially that lining the anterior dorsal meatus. There was focal destruction of the anterior nasoturbinate and nasal septum, local inflammation, and focal squamous metaplasia of the respiratory epithelium in rats and mice. Mild goblet cell hyperplasia was observed only in rats. The olfactory epithelium exhibited extensive loss of sensory cells with less damage to sustentacular cells. There was also loss of olfactory nerves, hypertrophy of Bowman's glands, and distension of the ducts of these glands by serocellular debris in regions underlying degenerating olfactory epithelium. At the 175-ppm exposure level, rats had more extensive olfactory lesions than mice, with hyperplasia of small basophilic cells adjacent to the basement membrane being present in rats but not mice. After 12 months of exposure to 10 ppm DMA, minimal loss of olfactory sensory cells and their axons in olfactory nerve bundles was observed in the nasal passages of a few rats and mice. These results indicate that the olfactory sensory cell is highly sensitive to the toxic effects of DMA, with minor lesions being produced in rodents even at the current threshold limit value of 10 ppm.