An exhaust emissions test program was conducted in Portland, Oregon on a sample of 292 privately-owned automobiles of 1975-77 model years. The objective of the program was to study the short-term emissions performance of cars that passed the mandatory Oregon emission inspection, in order to determine reasons for short-term emission control deterioration and ways to prevent it. Cars were recruited at the time they were due for inspection, tested in as-received condition and then at one, two, or three-month intervals. Cars failing the inspection were returned to the owners for repairs, then retested after passing the state inspection, and again after the one-to three-month interval. Cars showing significant emissions increases at that point were tuned to manufacturer's specifications by HTS and retested twice: immediately and after one month. Tests included the state idle test, the FTP, the Federal 3-mode test, and the Highway Fuel Economy Test. Complete engine diagnostic checks and owner interviews regarding maintenance and driveability were also conducted. Emissions and fuel economy data are presented for all tests.