Six normal young men were studied during 50 min of moderate exercise (100-137 W) that included one 15-min (protocol 1) or two 10-min periods of breathing 11-12% O2 (in N2) (protocol 2). Absolute work intensity was kept constant for each subject, but relative severity increased during hypoxia owing to reduction in maximum O2 uptake. Our question was whether hypoxia causes cutaneous vasoconstriction; this in turn should cause a rise in esophageal temperature (Tes) and a shift in the forearm skin blood flow (SkBF)-Tes relationship. In all subjects forearm blood flow (FBF) (venous occlusion plethysmography) rose throughout exercise and Tes tended to stabilize. Neither 10- nor 15-min periods of hypoxia caused systematic changes in FBF or Tes or their relationship to each other. We conclude that hypoxia equivalent to that incurred at 4,500-5,000 m does not significantly alter the short-term regulation of SkBF and body temperature during moderate exercise. Net cutaneous vasoconstriction is not elicited by arterial chemoreflexes under these conditions.