This paper discusses data reduction systems for use at exhaust emission test facilities. An investigation of ways and means of automatically reducing the large volumes of data generated in exhaust emission testing is discussed. Four distinct data reduction methods were considered and evaluated. The investigation resulted in the specification and purchase of an on-line data reduction system for use at several of the General Motors test facilities.Specifically, a system was developed to reduce data collected according to the California Exhaust Emission schedule. Data reduction is accomplished by electronic analog computation with digital control and output. Gas analyzers convert the hydrocarbon, carbon monoxide, and carbon dioxide composition of exhaust gas into analogous electrical signals. There are averaged by time scaled integration at specific times during the driving cycle. At the end of a given time the electrical analog data is operated on according to a specified equation. The accumulation of data during the schedule provides the information used for the final average values of HC and CO. The results are displayed on a chart recorder or digitized and typed on a report.