Endogenous adenosine can enter the extracellular space either by direct release or via formation from adenine nucleotides. We have compared the effects of substances blocking the ecto-5'-nucleotidase with a, b-methylene adenosine 5'-diphosphate (AOPCP) or adenosine kinase with iodotubercidin (Itu) on field potentials in the hippocampus and the neostriatum in vitro evoked by stimulation of the stratum radiatum or the cortico-striatal pathway respectively. AOPCP enhanced the amplitude of the population spikes by 34% in the hippocampus and by 26.5% in the neostriatum. DPCPX, a selective A1-receptor antagonist, increased the amplitude of the population spikes by 68% in the hippocampus and by 53.5% in the neostriatum. Thus both, release of adenosine from the intracellular space and, extracellular dephosphorylation of adenine nucleotides to a lesser extent, contribute to the effective levels of adenosine in the extracellular space in hippocampus and neostriatum.