The California Department of Transportation has applied regional air quality models to the Los Angeles area on a limited basis. A Lagrangian, or trajectory, model was used to evaluate the air quality impact of a highway project. An Eulerian, or grid, model is currently being used to evaluate regional transportation plans. The use of these models requires a costly and extensive data base for validation and the cooperation of local and regional agencies. A large aerometric sampling program was undertaken for one of the data bases. Technical problems in model application were related to electronic data processing and reconciliation of data that are tied to different geographic grid systems. In-house computer expertise is essential for application of the model. Model output tends to be voluminous and requires simplification for anlaysis and presentation./Author/