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“Central” and “peripheral” limitations to oxidative metabolism during exercise were evaluated in 10 young males following a 35-day horizontal bed rest (BR). Incremental exercise (IE) and moderate- and heavy-intensity constant-load exercises (CLE) were carried out on a cycloergometer before and 1–2 days after BR. Pulmonary gas exchange, cardiac output (Q̇; by impedance cardiography), skeletal muscle (vastus lateralis), and brain (frontal cortex) oxygenation (by near-infrared spectroscopy) were determined. After BR, “peak” (values at exhaustion during IE) workload, peak O₂ uptake (V̇o_2peak), peak stroke volume, Q̇_peak, and peak skeletal muscle O₂ extraction were decreased (−18, −18, −22, −19, and −33%, respectively). The gas exchange threshold was ∼60% of V̇o_2peak both before and after BR. At the highest workloads, brain oxygenation data suggest an increased O₂ extraction, which was …