Inhaled NO has become widely used for diagnosis and therapy of pulmonary hypertension. The potential hazards of NO inhalation include the formation of methemoglobin, formation of NO2, and generation of free radicals in the presence of humidity and oxygen. Careful monitoring of NO and NO2 concentration, and titration of the dose according to a patient's clinical response is essential to minimize toxicity. This paper describes a formula and method that permits calculation and precise control of NO concentration in the inspired gas. The accuracy of the delivery system was assessed by a comparison of calculated and measured NO and NO2 concentrations in a continuous flow ventilator circuit. A comparison of electrochemical detector (ECD) versus chemiluminescence detector (CLD) monitoring techniques showed agreement between the instruments within similar to 2 ppm, with the ECD averaging a higher reading than the calculated or CLD measured values. We deemed a 2 ppm discrepancy between instruments clinically acceptable, and concluded that the instruments could be used interchangeably for clinical purposes to measure NO, and that the ECD was preferable to CLD for measuring NO2, Details about the equipment are given and techniques are discussed to avoid the risk of inhalation of toxic concentrations of NO and NO2. This method provides the possibility of using inhaled NO with appropriate safety precautions in the range 0-60 ppm in a variety of continuous flow respiratory devices. (C) 1996 Wiley-Liss, Inc.