Muscular blood flow (MBF) was measured in the tibialis anterior muscle by the 133Xe washout technique. Measurements were made in lowlanders at sea level, in native highlanders at 3,800 m and in lowlanders who were living at this altitude for less than 6 mo. At rest and during exercise, MBF is smaller in highlanders and in the high altitude visitors than in lowlanders. If, instead of total blood flow, the red cell flow is calculated (by multiplying MBF by the hematocrit), these differences are no longer significant at rest and are minimized during exercise. These results are consistent with reduction of cardiac output and blood supply to other regions in subjects who are acclimatized to altitude.