Abstract: A method and appartus for determining patient lung pressure, compliance and resistance, for use in conjunction with a respirator system of the compensated-volume type wherein a predetermined volume of gas is delivered to the patient in each breathing cycle. The method and apparatus of the invention determine both the system pressure at the point of delivery to the patient and the patient lung pressure, both at the end of the inspiration phase of the breathing cycle, and utilize these values, together with a compensated volume signal, to compute the values of patient compliance and resistance. Control circuitry, actuated periodically or on demand, substitutes an enforced plateau period following inspiration, to allow the system pressure to fall to the value of lung pressure which is obtained at the end of the inspiration phase, thereby permitting the measurement of lung pressure by sampling system pressure at the end of the plateau period.