An electronically controlled respirator was constructed from 3 commercially available components: a positive-pressure ventilator, a recorder pen motor and a differential amplifier. Using negative feedback derived from a tracheal pressure signal, the instrument functions as a servo respirator which provides precise control of tracheal pressure. The system''s power and response characteristics are well suited for ventilation of anesthetized cats and dogs. The servo respirator can be used as an externally controlled respiratory pump which provides flexibility in selection of the parameters of the ventilatory cycle. Alternatively, it can function as a demand respirator which generates transthoracic pressure proportional to efferent respiratory discharge.