Body-weight-supported robot-assisted devices can be used to promote gait rehabilitation and as exercise tools for neurologically impaired persons such as stroke and spinal-cord-injured patients. Here, we propose a novel feedback-control structure for real-time control of oxygen uptake during robot-assisted gait, in which we use the following methods. 1) A feedback-control structure is proposed, consisting of a dynamic controller operating on target and actual levels of oxygen uptake in order to set a target work rate. Target work rate is achieved by an inner volitional feedback loop which relies on the subject's exercise input. 2) The dynamic oxygen-uptake controller is based on an empirically derived model of the oxygen-uptake dynamics and is synthesized by pole placement. 3) The resulting control system is tested during the robot-assisted treadmill ambulation of five able-bodied subjects. A single linear controller was designed based on identification data from tests with one subject and used for closed-loop control tests with all five subjects. In all cases, the actual oxygen-uptake response closely followed the ideal response as specified by the feedback design parameters. The control of oxygen uptake during body-weight-supported robot-assisted ambulation is feasible in the able-bodied population; the robustness of the system is demonstrated within the class of subjects tested. Further testing is required to validate the approach with neurologically impaired subjects.