Recent evidence has linked vehicle catalysts to nitrous oxide emissions. As the population of catalyst-equipped vehicles increases, so does the need for a better understanding of this powerful greenhouse gas.Nitrous oxide measurements were made on fourteen vehicles over the FTP urban and highway cycles utilizing an FTIR analyzer. To quantify emissions from aged catalysts, emphasis was placed on testing in-use vehicles with oedometer readings between 30,000 and 120,000 kilometres. Tailpipe N2O emissions averaged 126 mg/mile for the nine aged catalyst vehicles and 74 mg/mile for the five new catalyst cars. These results are about two times greater than previous N2O studies using the FTP cycle. Also, twice as much N2O was produced during the urban driving cycle, than during the highway cycle.Based on our measurements, N2O emissions from light duty vehicles in Canada were estimated to range from 8 to 20 kilotonnes for 1992. Pre- and post-catalyst measurements confirm that N2O is formed during the catalytic reduction process.