Local planners in Zarzal Valle del Cauca used census data and common sense to determine good centers from which to recruit rural health workers and at which to base ambulances. Their task can be formulated as a maximal covering location problem and was solved with both a heuristic appropriate to the local computer system and integer programming. The 7 factors to be used in selecting villages for puesto (health centers) are: promotoras (health promoters) should not have to travel more than 7.5 kilometers or more than 1 hour (whichever is less) 1 way between a health center and a client village; promotoras should not have to ascend more than 100 meters or descend more than 30 meters in travel between a health center and a client village; a health center should not be sited in towns with hospitals; a health center should have electricity service; a health center should have a portable water supply; a health center should not be established in a village which already has an existing health center; a health center should be sited in a village that is as accessible as possible to the unserved rural population. Mathematical programming terms were used to select health center sites. Maximal covering was also used to investigate incremental health center placement. Both the "initial siting" and the "incremental siting" solutions indicate a paradox associated with the use of electricity supply and acceptable water as requirements of health centers. Regardless of the particular standard which planners may select to guide rural health extension location analysis can assist them by inexpensively generating a list of sites that maximize the access of the population to promotora care either in unserved areas where no health centers exist or in underserved areas like Zarzal where more puestos will be added. Location analysis allows planners to both assess the performance of the current medical vehicle system and also determine good potential substitutes or additional sites for ambulances. Both general and specific recommendations regarding rural ambulance system have been adopted in Zarzal. The rural medical vehicles are now managed on a regional rather than a county basis. Maximal covering location analysis can be quicker less costly less idiosyncratic and more explicit than the judgement of planners. It allows people with different values and preferences to use the same information in making decisions.