The uneven distribution of professional health care providers is a worldwide problem. The distribution is skewed in favor of metropolitan urban residents, with far fewer providers available to rural populations. In 1936 Mexico instituted its “social service obligation” by which it required medical students to complete a 6-month tour of duty in a rural, underserved area as a condition of earning a medical degree. The Soviet and Mexican experiments provided guidance to efforts by other less developed countries seeking ways to extend professional primary care services to their rural hinterlands. In view of Mexico’s pioneering attempt to extend primary health care to undoctored rural populations through its social service requirement, and the virtual absence of evaluations of the success of this attempt, the Institute of Anthropological Investigation of the National Autonomous University commissioned a study of the program. In 1980, approximately 1,900 posantes were assigned to rural Mexican communities like San Francisco.