Abstract: An infant ventilator of the type that switches between an inspiration phase during which gas under pressure is applied through a mouthpiece to cause involuntary inspiration of an infant to which the mouthpiece is coupled, and an expiration phase during which the infant is allowed to exhale or voluntarily inhale through the mouthpiece while gas flows past the mouthpiece. The gas delivery system in the ventilator is interconnected so as to regulate the gas pressure at the mouthpiece during the expiration phase to generally atmospheric pressure (in the absence of operator selected PEEP) for any flow rate of gas past the mouthpiece in the expiration phase in the range of about 0 to 25 liters per minute as selected by a flow control valve without the need for further manual adjustments.