Adenosine and γ-aminobutyric acid (GABA) are both major inhibitory neuromodulators/neurotransmitters in the CNS. We now investigated if endogenous GABA modulates adenosine A 1 -mediated action on synaptic transmission in the hippocampus. Field excitatory postsynaptic potentials (fEPSP) were recorded from the CA 1 area of rat hippocampal slices. The adenosine analogue 2-chloroadenosine (0.15–1 μM) inhibited synaptic transmission with an EC 50 of 398 nM. Blocking GABA A receptors with the specific antagonists, bicuculline (10 μM) or picrotoxin (10 μM) potentiated the inhibitory effect of 2-chloroadenosine. The concentration–response curve for 2-chloroadenosine was displaced to the left by a factor of 2 (EC 50 =210 nM) in the presence of bicuculline (10 μM). GABA A receptor blockade also potentiated the action of N 6 -cyclopentyladenosine (CPA, 10 nM), a specific adenosine A 1 receptor agonist. Prevention of adenosine accumulation with adenosine deaminase (1 U/ml) did not influence bicuculline-induced potentiation of the effect of 2-chloroadenosine. The potentiation of adenosine A 1 -mediated response by bicuculline was abolished when nitric oxide (NO) synthase was inhibited with nitroarginine (100 μM), and when guanylyl cyclase was inhibited with 1H-[1,2,4]Oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 20 μM). The NO donors, (±)-S-nitroso-N-acetylpencillamine (SNAP, 300 μM) and diethylamine NONate diethylammonium salt (DEA/NO, 100 μM), significantly enhanced the inhibitory action of 2-chloroadenosine (150 nM). It is concluded that the blockade of GABA A receptors induces a potentiation of adenosine A 1 receptor-mediated inhibitory action, an effect that involves NO acting through guanylyl cyclase. Therefore, endogenous GABA might exert an inhibitory effect over adenosine A 1 -mediated responses in the hippocampus, which may represent a physiologic regulatory mechanism between the two inhibitory mediators.