A control system that maintains mean arterial pressure (MAP) at a desired level by automatically controlling halothane input has been developed. The control system is a discrete, digital type and is implemented with a small desk-top programmable calculator. Essentially, if MAP decreases, the inspired concentration of halothane will decrease; if MAP increases, the system will compensate by increasing inspired halothane concentration. The control system was tested in two ways: with a hybrid computer model of uptake and distribution of halothane and with dog experiments. The results indicate that (a) the model can be used to predict the response of the animal-anesthesia machine control system, and (b) the proposed algorithm is successful in controlling one anesthetic agent with one physiologic variable.