ABSTRACTObjective: To test the capillary recruitment hypothesis for the brain with control and hypoxic rats.Methods: Local cerebral blood flow (LCBF) was sharply raised by respiring 10% O2(hypoxia). LCBF as well as local influx rate constants (K1) and permeability‐surface area (PS) products of14C‐antipyrine and14C‐3‐O‐methyl‐d‐glucose (30MG) were estimated for capillary systems in 44 brain areas.Results: With this testing, an increase in PS product would be suggestive of capillary recruitment. In all brain areas, LCBF was increased by 30–90% by hypoxia. Hypoxia modestly raised the influx of antipyrine in brain but did not appreciably alter its PS products. With hypoxia,K1's and PS products of 30MG were significantly lowered (5–25%) throughout the brain, and the blood levels of glucose were sizeably raised. The latter increase would diminish the transfer of 30MG across the blood‐brain barrier by the hexose transporter because of increased glucose competition. By applying a glucose‐concentration correction to the data, the apparent PS product of 30MG for the hypoxic group became equal to that for the controls, which agrees with the antipyrine PS product results.Conclusions: Hypoxia, thus, leads to virtually no increase in PS products and no capillary recruitment in brain, and elevates LCBF mainly, perhaps exclusively, by raising the velocity of flow through already