Extracellular adenosine in the striatum of adult male rats was measured by the use of a microdialysis fibre inserted transversely in the striatum. The adenosine concentration in samples of perfusate was determined by HPLC coupled to U.V. detection. The adenosine concentration (corrected for recovery) decreased after implantation of the probe. Two hours later it was 1.83 +/- 0.22 in anaesthetized rats, whereas it was 40% higher in rats in which anaesthesia had been discontinued. Twenty-four hours later the adenosine concentration was 0.124 +/- 0.09 microM; the addition of dipyridamole (100 microM), an adenosine uptake blocker, to the perfusate resulted in a 76% increase in adenosine concentration in the effluent, whereas addition of the adenosine deaminase inhibitor erythro-2-(hydroxy-3-nonyl) adenine (100 microM) caused a 260% increase. The addition of tetrodotoxin (1 microM) was followed by a decrease in basal adenosine concentration and a partial inhibition of the increase in adenosine evoked by K+ depolarization. The increase induced by high K+ was markedly inhibited by the NMDA receptor antagonist D(-)-amino-7-phosphoeptanoic acid (1 mM, D-AP7). These findings indicate that the extracellular adenosine level is influenced by neuronal activity, and that under strong depolarizing conditions the increase in adenosine level involves NMDA receptor activation.