A description is given of a DYNAMO-based model, ENTRANS, which simulates the basic interrelationships between energy supply and transportation-related energy use. The model was developed to facilitate analysis of the impacts on energy supply of different transportation policies, and of the effects on transportation of energy supply policies. ENTRANS consists of several linked sectors. The highway and transit sectors relate transportation service (travel times and costs) to use levels and to government capital inputs. The auto sector traces consumer purchase patterns, auto industry production, and auto fleet characteristics as functions of gasoline price and transportation service levels. The model system is capable of forecasting the effects of a wide range of transportation and energy policies.