The new concept of mandatory minute volume (MMV) is described. The system provides a preset minute volume to the patient, who breathes spontaneously from it as much as he is able, the remainder being delivered to him via a ventilator. The necessary apparatus has been constructed and has the additional facility of PEEP and/or CPAP up to a level of 15 cmH2O pressure. With possible exceptions, the apparatus allows simpler and more direct control over the patient's PaCO2 than with the IMV system and it ensures, without adjustment, a constant minute volume of fresh gas breathed by the patient, despite minute-to-minute changes in his ability to breathe. Use of the system in conjunction with two commonly used ventilators is described. It should not be difficult to build the facility for MMV into new versions of artificial ventilators.