The effects of carbon monoxide (CO) on blood carboxyhemoglobin (COHb) levels, though well studied at sea level, have not been investigated in populations at altitude. COHb levels were measured in laboratory rats following 6 wk exposure to either clean air or air containing 9 ppm CO at ambient altitude (3300 ft), 10,000 ft, or 15,000 ft simulated high altitude. In animals breathing clean air, COHb levels increased with increasing altitude from 0.68 ± 0.09% at 3300 ft to 1.16 ± 0.28% and 1.68 ± 0.14%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude is linear with a correlation coefficient of 0.90 (p < .001). In animals breathing 9 ppm CO, COHb levels also increased with increasing altitude from 0.99 ± 0.06% at 3300 ft to 1.77 ± 0.17% and 2.10 + 0.08%, respectively, at 10,000 and 15,000 ft. The relationship between COHb levels and increasing altitude in animals breathing CO is also linear with a correlation coefficient of .92 (p < .001). These data indicate that, compared with animals at sea level, animals at altitude have an increased body burden of COHb and will attain the COHb level associated with the National Ambient Air Quality Standard for CO more quickly when breathing CO.