Apparatus (1) for relieving dyspnoea in an ambulatory exercising subject comprises a housing (3) to be worn on the subject, and within which a variable speed air blower motor (5) is located for delivering an air supply to the subject through a mouthpiece (10) at a pressure similar to average intrinsic positive end-expiratory pressure of the subject. A pressure transducer (25) in the mouthpiece (10) monitors the pressure during breathing cycles of the subject, and a control circuit (22) determines the average intrinsic positive end-expiratory pressure of the subject over five breathing cycles. The control circuit (22) operates the air blower motor (5) for delivering the air supply to the subject at a pressure similar to the determined average intrinsic positive end-expiratory pressure.