This paper describes a simulation study of the performance of an intelligent control system designed to adjust the ventilator respiratory rate during assist-control mechanical ventilation. The system described utilizes fuzzy logic to facilitate the inclusion of expert knowledge from clinicians in the form of linguistic statements. The system's ability to make appropriate adjustments was demonstrated in seven computer simulated patient-ventilator gases. The controller took appropriate action in all cases. Intelligent control architectures may constitute an important new approach to closed-loop decision support. Intelligent control of ventilator respiratory rate settings in assist-control ventilation is a candidate for further research and clinical validation.