Study of the mechanical properties of the respiratory system is needed to help provide a better understanding of the pathogenesis of diseases causing respiratory failure. The nature of neonatal intensive care requires that any technique for monitoring respiratory mechanics be simple, noninvasive, and allow continued free access to the neonate. The peak airway pressure developed during volume cycled ventilation reflects the mechanical properties of the respiratory system but cannot distinguish between changes in the flow-resistive or elastic properties. Similarly, dynamic compliance combines both the flow-resistive and elastic components of the respiratory system in a single number and flow-volume loops also reflect both elements. Extracting a single time-constant from the expiratory limb of the latter assumes a single-compartment model for the respiratory system and, as such, does not provide sufficient information to describe frequency dependence of resistance and compliance. Furthermore, flow-volume loops are markedly distorted by the presence of an endotracheal tube, which must be corrected for, before calculating values of resistance and compliance. To provide the information to understand better the physiologic processes and adaptive mechanisms in diseased states causing acute respiratory failure, it is necessary to use a method that is based on a more detailed and realistic model of the respiratory system. Two such techniques that appear to warrant further investigation in ventilated infants are the interrupter technique and the forced-oscillation technique.