The exceptional ability of the turtle brain to survive prolonged anoxia makes it a unique model for studying anoxic survival mechanisms. We have used epi-illumination microscopy to record blood flow rate in venules on the cortical surface of turtles (Trachemys scripta). During anoxia, blood flow rate increased 1.7 times after 45-75 min, whereupon it fell back, reaching preanoxic values after 115 min of anoxia. Topical superfusion with adenosine (50 microM) during normoxia caused a 3.8-fold increase in flow rate. Superfusing the brain with the adenosine receptor blocker aminophylline (250 microM) totally inhibited the effects of both adenosine and anoxia, while aminophylline had no effect on normoxic flow rate. None of the treatments affected systemic blood pressure. These results indicate an initial adenosine-mediated increase in cerebral blood flow rate during anoxia, probably representing an emergency response before deep metabolic depression sets in.