We describe a new method of measuring respiratory system compliance (Crs) that appears to detect whether respiratory muscles are relaxed. A pulse airflow, 0.3/s, is blown into the mouth through a pneumotachograph. Mouth pressure is recorded on the abscissa of an oscilloscope. Inflated volume, integrated from the flow signal, is recorded on the ordinate. After an initial step shift of pressure related to the flow resistance of the subject, pressure increases linearly at a rate inversely proportional to Crs. Crs is calculated from the slope of the volume-pressure line. With relaxed subjects, repeated pulses yield straight lines with similar slopes (mean coef of variation = 4.8%). When subjects are not relaxed, the pulse produces irregular lines. In 15 normal subjects who could relax. Crs averaged 0.86 +/- 0.016 (SD) 1/cmH2O. When normalized for body size by dividing by the vital capacity, the mean value was 0.021 +/- 0.0024 cmH2O-1, which agrees with published values. We conclude that the pulse method accurately measures Crs and has the advantage of detecting respiratory muscle relaxation.