Abstract Measurements with the Wells-Brookfield cone-plate microviscometer revealed no significant differences in blood viscosity between controls and female beagles exposed to laboratory produced air pollutants daily for 4 years, despite significant elevations of carboxyhemoglobin in four treatment groups. Air pollutant treatments included: raw auto exhaust (RAE), irradiated auto exhaust (IAE), oxides of sulfur (SO x ), RAE + SO x , IAE + SO x , “low” nitric oxide + “high” nitrogen dioxide, “high” nitric oxide + “low” nitrogen dioxide. The control treatment was temperature- and humidity-conditioned, filtered clean air.