Apparatus for the supply of breathable gas cyclically at an inspiratory pressure and a lower expiratory pressure substantially in synchronism with a patient's respiration. A flow generator coupled to a gas delivery system delivers breathable gas to the patient's airway. A motor controller receives a signal representing respiratory flow. The motor controller sends out a signal to control an electric motor and turbine to generate the desired inspiratory and expiratory pressures. The controller detects transitions between inspiration and expiration via a flow signal to discriminate between the respiratory phases of the patient. A counter counts a first time duration commencing from the last transition to inspiration and whereby if the first time duration elapses before a transition to expiration is detected, the controller initiates the supply of a given expiratory pressure. Furthermore, a second counter counts a second time duration commenced from the last transition to inspiration detected and signals the turbine to supply a given inspiratory pressure until the second time duration has elapsed even if during the second time duration a transition to expiration by the patient has occurred.