The release of adenine nucleotides and adenosine from the in vivo rat cerebral cortex has been measured in the presence of DL-alpha-glycerophosphate (10(-2) M), an inhibitor of non-specific phosphatases. In normoxic brains, the superfusate concentrations of adenosine and its nucleotides (in nM) were: adenosine 38.9 +/- 10.9; AMP 13.9 +/- 1.6; ADP 12.4 +/- 2.3; ATP 17.7 +/- 2.1. During 10 min episodes of hypoxia (8% oxygen inhalation) superfusate levels of adenosine doubled, but the release of nucleotides was unaltered. These findings indicate that extracellular levels of ATP in the cerebral cortex are in the range at which this compound dilates cerebral blood vessels, and that ATP may be an endogenous regulator of regional cerebral blood flow. The absence of any increase in nucleotide levels during hypoxic episodes implies that adenosine is released into the extracellular space as the nucleoside, and not as a nucleotide which would then be hydrolyzed to adenosine.