As large medical centers become ever more capable, the existence of well equipped, well staffed, and rapid emergency ambulance service becomes increasingly important. This paper presents a method for determining the optimum location of ambulance stations to minimize the average response time to emergency calls. A new point-to-point driving time model is introduced, and a computer optimization algorithm used to determine the optimum locations. A constraint that the average response time to any point in the service area be less than some specified minimum is also considered. The method is applied to Washtenaw County, Michigan.