We hypothesized that forearm blood flow (FBF) during moderate intensity dynamic exercise would meet the demands of the exercise and that postexercise FBF would quickly recover. In contrast, during heavy exercise, FBF would be inadequate causing a marked postexercise hyperemia and sustained increase in muscle oxygen uptake (V˙o _2musc). Six subjects did forearm exercise (1-s contraction/relaxation, 1-s pause) for 5 min at 25 and 75% of peak workload. FBF was determined by Doppler ultrasound, and O₂ extraction was estimated from venous blood samples. In moderate exercise, FBF andV˙o _2musc increased within 2 min to steady state. Rapid recovery to baseline suggested adequate O₂supply during moderate exercise. In contrast, FBF was not adequate during heavy dynamic exercise. Immediately postexercise, there was an ∼50% increase in FBF. Furthermore, we observed for the first time in the recovery period an increase inV˙o _2musc above end-exercise values. During moderate exercise, O₂ supply met requirements, but with heavy forearm exercise, inadequate O₂ supply during exercise caused accumulation of a large O₂ deficit that was repaid during recovery.