The automated breathing metabolic simulator (abms) is a machine capable of performing almost any physiologically appropriate respiratory simulation. Since the first abms was delivered to the Bureau of Mines in 1973, efforts have been made to refine the systems and components that make up the abms, including the breathing simulator module, the gas analysis module, and the supervisory controller, each of which is capable of stand-alone operation. To improve the equipment and to develop simulation criteria and parameters, testing and modification of the abms has been continuing since 1980. Changes have been made to the phase i design plan and performance specifications. Future improvements based on current experience are recommended, and test results of full-scale testing conducted in November 1983 on several types of breathing apparatus are reported and illustrated.