A method and apparatus for detecting NO.sub.x by chemiluminescence. The apparatus includes a sample gas line having an inlet port for receiving a sample gas, a NO.sub.x converter connected to the sample gas line, a N.sub.2 diluting gas line connected to the sample gas line upstream from the NO.sub.x converter, a bypass exhaust lie connected to the sample gas line, and a chemiluminescence analyzer connected to the NO.sub.x converter. The sample gas, which contains a NO.sub.x component, is introduced to the sample gas line through an inlet portion. The sample gas is diluted with nitrogen gas from the nitrogen diluting gas line, and the NO.sub.x component in the sample gas is converted to NO by the NO.sub.x converter. The chemiluminescence analyzer detects the NO.sub.x component.