The present invention relates to systems and methods for administering high concentrations of nitric oxide (NO) gas to a patient without the need to provide supplemental oxygen to the patient. The systems and methods can be used to administer high therapeutic amounts of NO gas, for example a gas comprising 160 ppm NO, while forming little or no residual nitrogen dioxide (NO2). The method is based on using a NO gas source with a relatively high NO concentration, for example 5,000 ppm, while rapidly mixing the gas from the NO gas source with air immediately prior to administering the gas to a patient.