1. A method of controlling operation of a lung ventilator, which includes a source of breathing gas having means capable of delivering a volume of breathing gas per unit of time; which is at least equal to the maximum volume that may be required for the ventilation of the patient, an inspiratory line having means for connecting the same to the airways of a patient and through which the patient can breathe spontaneously from said breathing-gas source, a ventilator unit connected to said breathing-gas source and activateable by an activation signal to deliver to the patient through said inspiratory line a mandatory breath of a predetermined tidal volume from said breathing-gas source, and an expiratory line having means for connecting the same to the airways to the patient and through which the patient can exhale, the method comprising the steps of sensing continuously the flow rate of breathing gas through said inspiratory line and generating a signal proportional to the momentary value of said flow rate, producing a reference signal substantially proportional to the minimum volume per unit of time, the so called minute volume, of breathing gas which the patient shall receive, said minimum volume per unit of time being less than the volume that the breathing gas source is capable of delivering, calculating continuously the total integral, during the entire inhalation and exhalation cycles of ventilation of the patient, of the difference between said reference signal and said flow rate signal and generating continuously a signal proportional to said integral, producing a first threshold signal substantially proportional to said tidal volume of a mandatory breath, comparing continuously said signal proportional to said integral and said first threshold signal, and producing an activation signal for activating said ventilator unit to deliver a mandatory breath of said predetermined tidal volume to the patient each time the value of said signal proportional to said integral reaches the value of said first threshold signal.