A system has been designed which allows for measurement of cardiorespiratory parameters in the fully submerged subject performing graded exercise. It consists of a horizontal wet chamber; a waterproofed, electrically braked bicycle ergometer; and a low-resistance "bag-in-a-box" breathing apparatus. Chamber and breathing apparatus design allow for a great deal of flexibility in both positioning of the subject and instrumentation. The 200-liter "bag-in-a-box" configuration provides the subject with humidified gas through 2.5 in. i.d. tubing. A rolling seal spirometer provides the lung counter volume. Provision is made for breath-by-breath gas analysis with a mass spectrometer. Hydrostatic pressure on the diver's thorax relative to chamber pressure can be easily and reproducibly varied over a wide range of positive and negative static lung loads. This system has been used on over 100 man-dives to depths equivalent to 6.5 ATA with oxygen consumptions up to 4.0 liters/min.