This article describes a new technique for estimating the availability of physician services in small geographic areas. Given counts or estimates of the number of physicians practicing in small geographic areas (e.g., zip codes), the technique allocates a portion of the services of each physician in an area to his home area and to nearby areas in proportion to both the propensity of patients to travel for medical care and the availability of potential patients. The longer the time required for a patient to travel to the physician, the smaller the proportion of the physician's services allocated to the patient's area, with the precise relationship determined by a special analysis. The final estimate of the availability of physician services in each small area is the sum of the service proportions of every physician in all of the small areas. The total supply of physician services is the same as the original total, but the distribution is adjusted to reflect the time that patients are willing to spend traveling to obtain medical care. Although this technique requires considerable data processing, it permits more accurate estimation of the supply of physician services in small geographic areas than is possible with traditional methods. It better represents the probabilistic and interpenetrating nature of physician service areas than alternative techniques and appears to be particularly applicable in estimating the supply of primary care physician services. Actual data for pediatricians and children in northeastern New York using zip codes as the geographic units illustrate the technique. Limitations, applications, and possible extensions are discussed.