An automated animal inhalation exposure facility was designed to conduct chronic exposures of rodents to gaseous pollutants. The facility consisted of 3 walk-in chambers with modular cages to allow for exposure of up to a maximum of 480 mice or 240 adult rats. Critical parameters of operation, such as relative humidity, pollutant concentration, and temperature, were monitored continuously. Failure of the system to maintain parameters within specified limits activated the alarm system. Distribution of test gases in the chambers was evaluated for homogeneity, and the standard deviation was determined to be within + or - 5 percent of the target concentrations throughout the chamber. The exposure facility was successfully used to expose 720 rats to diurnal patterns of O3 (baseline of 0.06 ppm for 13 h with an exposure peak to 0.25 ppm over 9 h) and NO2 (baseline of 0.5 ppm for 16 h with an exposure peak to 1.5 ppm over 6 h) for a period of 18 mo. (Copyright (c) 1987 by Hemisphere Publishing Corporation.)