The accessibility, distribution and utilisation of emergency medical services are important components of health care delivery. The impact of these services on well-being is heightened by the fact that ambulance resources must respond in a reliable and timely manner to emergency calls from demand areas. However, many factors, such as the unavailability of an ambulance at a center closest to a call, can adversely influence response time. This paper discusses the design and implementation of a framework developed in a Geographic Information System for assessing ambulance response performance. A case study of ambulance response in three communities in Southern Ontario, Canada is presented that allows easy and rapid identification of anomalous calls that may adversely affect overall operating performance evaluation. Extensions of the framework into a fully fledged service deployment and planning decision support system are discussed.