Abstract: Apparatus for exercising the muscles of the respiratory system for improving endurance. The apparatus includes a bag which is inflatable for receiving a predetermined volume of expired air. A valve is closed during rebreathing of expired air in the bag and is adapted to open in response to deflation of the bag to admit fresh air to the air way. The valve is also adapted to open when the bag is fully inflated to release excess expired air. The breathing frequency is paced so that, in combination with the predetermined volume, the overall ventilation may be maintained over a period of time for increasing endurance. As endurance is increased over time, the bag may be replaced with incrementally larger volume bags or the bag volume may be incrementally increased. Guidance of the training schedule and central monitoring of the training are also provided.