Abstract: Apparatus for artificial pulmonary ventilation during anaesthesia and resuscitation in which the gaseous transfer occurs by diffusion. The apparatus which is highly reliable, has a long working life, and achieves more active alveolar exchange, includes an on-off valve in a line for supplying air to the lungs. The valve has a valve body, a shutter seat in the body, respective air inlet and outlet ports in the valve body which open into the seat, a shutter sealingly rotatable in the seat under the action of drive means, and at least one duct in the shutter for interconnecting the ports periodically in accordance with the rotation of the shutter.