A new spectroscopic technique is proposed for the remote detection and measurement of the gas nitric oxide. Radiation that has been emitted or absorbed by the sample of gas under investigation is incident on a radiometer, within the optical system of which is included a cell containing nitric oxide. This cell is surrounded by a magnetic field whose amplitude or polarization properties can be modulated. Radiation at the modulation frequency is selected by the detector, thus providing detection of NO in the sample with high sensitivity and high selectivity. Comparison is made between the performance of a laboratory instrument with theoretical prediction. The radiometer is particularly suitable for the observation of nitric oxide in the atmosphere.