Mass spectrometers should have unique advantages in the evaluation of VA/Q,  inequalities in neonatal respiratory distress syndrome (RDS), especially, in neonates requiring assisted ventilation. The purposes of this report are to (7) describe a method for continuous measurement of respired gas composition utilizing a mass spectrometer system, and (2) describe the results of serial aADC02 and AaD02 (expressed as Qs/Q) measurements in 63 neonates with RDS. The mass spectrometer system utilized is a mobile unit which includes the following components: quadrupole mass spectrometer for gas analysis, flow meter, and an analogue computer for measurement of respiratory flows and volumes. In 17 patients not requiring assisted ventilation, initial aADC02 levels ranged from normal (<5) to 26 mm Hg; there was considerable variation in initial Qs/Q In 46 patients requiring assisted ventilation, Qs/Q measurements did not distinguish pulmonary from nonpulmonary deaths or survivors from nonsurvivors. Serial aADG02 measurements, however, indicated a correlation between ability to achieve an aADC02 < 10 mm Hg by age 7-10 days and potential for pulmonary survival. Full utilization of a mass spectrometer system will allow for breath-by-breath measurement of AaDO2, CO2, and N2, as well as a variety of respiratory flows and volumes. Such measurements should provide further understanding of A, inequalities in RDS. Serial aADCO2 levels may provide a more objective measurement of acute severity as well as a better definition of potential for pulmonary survival.