The finding that catalytic converters enhance the N2O emissions from motor vehicles give rise to the concern that they may represent a significant source of N2O. Measurements of N2O and CO2 emissions of a fleet of about 40,000 cars were conducted in two roadway tunnels in Stockholm, Sweden, and Hamburg, Germany, to estimate N2O emissions under normal driving conditions. To avoid any risk of artifact N2O formation, samples were analyzed within 1 min after collection. Median N2O to CO2 molar emission ratios of 1.4¿10-4 and 0.6¿10-4 were measured in Stockholm and Hamburg, respectively. The lower emission ratio in Hamburg may well reflect the smaller fraction of catalyst-equipped cars in Germany (22%) compared to the Swedish vehicle fleet (33%) as well as low N2O emissions from trucks which were not allowed in the Stockholm tunnel. From our measurements we derive a N2O to CO2 molar emission ratio from cars equipped with catalysts in the median to interquartile range of 3.8¿2.2¿10-4, from which present worldwide N2O emissions from cars are estimated to be 0.38¿0.22 Tg N2O (0.24¿0.14 Tg N) per year, corresponding to 3--16% of the atmospheric growth of 4.7¿0.9 Tg N2O/yr (3.0¿0.6 Tg N/yr). From the worldwide emission of 0.38 Tg N2O we derived an emission of about 170 mg N2O/km for catalyst-equipped cars, significantly higher than those reported in earlier studies. If the entire present fleet of cars were to be equipped with current types of catalysts, the global N2O emission could double and reach 6--32% of the atmospheric growth rate. ¿ American Geophysical Union 1993