Rapidly growing cities cause inadequate transformation in the use of land. Government policies tend to be obsolete shortly after implementation, supply of basic services becomes insufficient and expensive, and food prices tend to increase. All these factors are certainly the case in many developing countries where large amounts of population often migrate to settle down, sometimes in dangerous or unhealthy locations, but other times in areas suitable for agricultural purposes in the periphery of the city.The land surrounding the metropolis enters a transitional stage. The territory becomes uneconomical for rural exploitation, but it requires to be supplied with basic services for urban housing. The System Dynamics approach is then appropriate to study and plan these unstable systems.This paper presents a model to assess the growth of peripheral districts of the city. It is a useful aid for policy making in land use issues and a good tool for planning basic services such as health, schooling, transport and recreation.A simulation is carried out for the Periurban District of El Corazon in Medellin, Columbia. A good approximation between historical data and model results can be appreciated. Some scenarios of future growth are explored and the consequence of land-use policies are confronted.