Inhalation of chlorine (Cl2) or formaldehyde (HCHO) stimulates the trigeminal nerve endings in the nasal mucosa and results in respiratory rate depression in a concentration-dependent manner. To determine tolerance and cross-tolerance, the concentration-response curves of respiratory depression were compared between naive rats and rats pre-exposed to Cl2 or HCHO. Chlorine tolerance development was time and concentration dependent, being significant following a 1-day (6 hr/day), 10 ppm exposure, and reaching the maximum in 4 days. At 2.5 ppm of Cl2, tolerance was significant only after 10 days of exposure. Rats tolerant to Cl2 also showed cross-tolerance to HCHO. Tolerance to HCHO was observed in rats exposed to 28 ppm for 4 days, but not in groups exposed to 15 ppm for 1, 4, or 10 days. However, significant cross-tolerance to Cl2 was evident following a 1-day exposure to 15 ppm HCHO, with greatest effect seen in the group exposed for 10 days. Tolerance was reduced after a 7-day recovery following a 4-day exposure. Cross-tolerance was reduced also, but to a much lesser extent. These results suggest a common mechanism for tolerance and cross-tolerance development, but different reactive sites may exist for Cl2 and HCHO at the trigeminal nerve endings.