A Siemens Elema 900B servo ventilator was modified to adjust ventilation automatically in real time. The end tidal PCO2was monitored continuously and a microcomputer system was used to adjust minute ventilation on a breath-by-breath basis. The system was tested in three dogs during HCI infusion. With the loop open (constant ventilation), end tidal PCO2increased. When the loop was closed, the microcomputer made adjustments to the delivered minute ventilation to maintain the end tidal PCO2at the preset value. This system could find applications in basic physiological research as well as in humans with further testing and evaluation.