This paper introduces the Anesthesia Response Simulator (AReS), a new, open-source tool designed to mimic patient physiological responses to four common anesthesia drugs: propofol, remifentanil, norepinephrine, and rocuronium. The simulator integrates established drug models with expert clinical knowledge to model key vital signs and the effects of surgical stimuli. To validate its accuracy, the simulator’s outputs were compared against clinical data from 10 patients. The results showed that the simulator accurately replicated clinical trends for most patients, and its responses to drug infusions and simulated disturbances were consistent with existing medical literature and clinical practice. We conclude that this open-source simulator is a valuable tool for developing and testing new automated anesthesia controllers and decision support systems, while also noting its current limitations and suggesting areas for future improvement.