A method and apparatus are provided for reducing pollutants and especially nitrogen oxides, or NOx, in the exhaust gases of an engine. The apparatus includes a device for injecting ammonia into a conduit that extends between cylinder exhaust valves and a catalytic converter, to mix the ammonia with the exhaust gases. Ammonia injection occurs at a location a plurality of inches downstream of the exhaust valves to avoid burning of the ammonia by flames. The injection preferably occurs at a location where the exhaust gases are at a temperature of at least about 1200.degree. F. to cause considerable reaction of the ammonia with NOx in the exhaust gases prior to the gases reaching the catalyst, so less NOx must be removed along the catalyst. To minimize ammonia use, it is injected only for a limited period such as a minute after a cold engine is started when the catalyst is too cold to cause significant reactions, and during operation of the engine at high power levels when the greatest amounts of NOx are produced.