A system has been developed to measure simulatneously the effects of inhaled toxicants on cardiopulmonary function in four awake rats before, during and after exposure. One day prior to testing, Fischer-344 and Sprague-Dawley rats were implanted with an intrapleural or carotid catheter. On the day of testing, the rats were placed into a restraint system to reduce movement artifacts and to keep rats from disturbing the implanted catheters without imposing limitations on functional measurements. The rats were then sealed into constant-volume plethysmographs mounted on the door of a 0.32-cu m Rochester exposure chamber. Thirty minutes after the rats were placed into the plethysmographs, control measurements were obtained. Under the control of a digital computer, the rats were exposed for 15-min periods to 4% CO2. During testing, tidal volume, intrapleural pressure, blood pressure, and the electrocardiogram were recorded on the polygraph, and were digitized and stored in the computer. Four-second breathing epochs were recorded and analyzed, and the results were displayed on line.