1. In the present work, we investigated the action of adenosine originating from extracellular catabolism of adenine nucleotides, in two preparations where synaptic transmission is modulated by both inhibitory A1 and excitatory A(2a)-adenosine receptors, the rat hippocampal Schaffer fibres/CA1 pyramid synapses and the rat innervated hemidiaphragm. 2. Endogenous adenosine tonically inhibited synaptic transmission, since 0.5-2 u ml-1 of adenosine deaminase increased both the population spike amplitude (30 +/- 4%) and field excitatory post-synaptic potential (f.e.p.s.p.) slope (27 +/- 4%) recorded from hippocampal slices and the evoked [3H]-acetylcholine ([3H]-ACh) release from the motor nerve terminals (25 +/- 2%). 3. alpha, beta-Methylene adenosine diphosphate (AOPCP) in concentrations (100-200 microM) that almost completely inhibited the formation of adenosine from the extracellular catabolism of AMP, decreased population spike amplitude by 39 +/- 5% and f.e.p.s.p. slope by 32 +/- 3% in hippocampal slices and [3H]-ACh release from motor nerve terminals by 27 +/- 3%. 4. Addition of exogenous 5'-nucleotidase (5 u ml-1) prevented the inhibitory effect of AOPCP on population spike amplitude and f.e.p.s.p. slope by 43-57%, whereas the P2 antagonist, suramin (100 microM), did not modify the effect of AOPCP. 5. In both preparations, the effect of AOPCP resulted from prevention of adenosine formation since it was no longer evident when accumulation of extracellular adenosine was hindered by adenosine deaminase (0.5-2 u ml-1). The inhibitory effect of AOPCP was still evident when A1 receptors were blocked by 1,3-dipropyl-8-cyclopentylxanthine (2.5-5 nM), but was abolished by the A2 antagonist, 3,7-dimethyl-1-propargylxanthine (10 microM). 6. These results suggest that adenosine originating from catabolism of released adenine nucleotides preferentially activates excitatory A2 receptors in hippocampal CAI pyramid synapses and in phrenic motor nerve endings.