The paper describes a development of the Urban Dynamics model into a spatially-disaggregated model. The resulting model is submitted to a number of tests and policy changes. An extension of the model to simulate urban spread and development without the assumption of an a priori fixed area is also discussed. The paper goes some way to providing a synthesis of the Urban Dynamics model with the static but spatially disaggregated Lowry model, the forerunner of the most widely used type of land use model. The flexibility of the system dynamics method may lead one to overlook certain problems of specification and calibration, three of which are illustrated using the Urban Dynamics model. © 1979.