Extracellular adenosine and ATP levels were monitored by microdialysis in the striatum of the freshwater turtle Trachemys scripta during long-term N2 respiration. After an initial rise in extracellular adenosine, a second peak of longer duration and higher in intensity, followed. The frequencies of these adenosine cycles varied considerably between individual turtles, such that the shortest time between the peaks was 80 min and the longest was 300 min (mean 151 min). After about 60 min anoxia, there was also a slow increase in extracellular ATP, rising from a normoxic concentration of 1.21 +/- 0.12 to 7.58 +/- 3.70 nmol l(-1) at 240 min anoxia. The results suggest that adenosine may continue to have a protective function in the turtle brain during long-term anoxia and that extracellular ATP might not function as an excitatory neurotransmitter in the anoxic turtle brain.