Infants with respiratory distress syndrome are treated with constant distending pressures to counter atelectasis and increase the functional residual capacity (FRC). However, FRCs are not monitored in these severely ill infants because present methods are impractical. We computerized N2-washout techniques for estimating the lung volume in these infants. The system has been evaluated using 1) a mechanical lung model and 2) small cats with normal and edematous lungs. Comparing the results of measured lung volumes with the fixed mechanical lung volumes yielded a correlation coefficient 0.996 (n=50). Decreasing the N2 gradient between initial N2 concentration and the washout medium from 0.79 to 0.10 did not affect the accuracy or reproducibility of measurements in the mechanical lung. For normal cats, comparing the results of N2-washout measurements of FRC with He-dilution values yielded a correlation coefficient of 0.921 (n=128); results from cats with pulmonary edema yielded a correlation coefficient 0.989 (n=80). We conclude that this computerized system is accurate, reproducible, and clinically acceptable for monitoring infants.