As physicians and other providers of health care services see their traditional markets erode, an increasingly important element of any provider location decision is the determination of a population base or "critical mass" that can professionally and financially support a given set of health care services. While the size of a local population is not the sole determinant of success, ultimately an adequate population base to support a given spectrum of services must be defined, and providers increasingly need tools for evaluating opportunities in the new economic market. This is especially true in rural areas. An earlier supply and demand model for estimating the critical mass of population needed to support a physician in any one of 25 specialties and subspecialties in urban and suburban areas is adapted to the rural market. The assumptions inherent in the earlier model are examined and the issue of "critical mass" is examined from a rural health care perspective in this paper.