The constant-flow properties of time-cycled ventilators can be used to measure infant respiratory mechanics. We used a pulse method that does not interrupt inflationary flow to measure lung compliance (Cl) and respiratory system compliance (Crs) of 16 infants who required assisted ventilation. When the infants were relaxed, constant-flow inflation produced transrespiratory pressure tracings with constant slope segments. We calculated pulse Crs from inflationary flow divided by the slope of the pressure tracing, and compared the results to static and dynamic Crs values determined by standard methods. The pulse method accurately measured static Crs (r = .93) with a low intrasubject coefficient of variation (3.4%). Pulse and static Crs values consistently exceeded dynamic Crs (p less than .005). Cl measured with each method exceeded Crs (p less than .05), but the magnitude was clinically unimportant. Pulse Crs is a noninvasive measurement of static respiratory system recoil which proved to be sensitive to changes in respiratory muscle tone.