The methods presently available for measuring FRC of newborn infants are impractical and used primarily as research tools. This report describes a new device for estimating FRC which is accurate, reproducible, and practical for use on critically ill infants in the intensive care unit. Measurements are made using either [1] standard multiple breath N2 washout or [2] a new 4-breath N2 washout method of estimating FRC. Automating the measurements with a microcomputer reduced the time required for data reduction from 45 min to seconds. Utilizing the 4-breath method reduced measurement times (including time for reequilibration) from 15 min to less than a minute; thus, repeated measurements may be made even in critically ill infants suffering from RDS whose FRC may be relatively unstable. Breathing O2 for only 4 breaths also reduces possible damaging effects of O2 such as retinopathy and absorption atelectasis.