Previous work has been carried out on the effects of adenosine on transmitter release and on the excitability of postsynaptic neurones, but little is known about the effects of adenosine on the coupling between the two. In this study, we examine the effects of specific adenosine receptor agonists and antagonists on the population excitatory postsynaptic potential (population EPSP) slope, the population spike amplitude, and the relationship between the two (E-S coupling) in the CA1 area of rat hippocampus. Activation of adenosine A1 receptors by adenosine or the selective agonist N6-cyclopentyladenosine resulted in a decrease of the population spike amplitude by a greater extent than could be accounted for by the decrease in population EPSP slope, resulting in a dissociation in the E-S relationship, reflected as a right-shift in the E-S curve. Activation of adenosine A2A receptors by the selective agonist 2-p-(2-carboxyethy)phenethylamino-5'-N-ethylcarboxamidoadeno sine (CGS 21680), or blockade by antagonists ZM 241385 and CP 66713 had no effect on evoked responses. However, when both adenosine A1 and A2A receptors were activated at the same time, a significant attenuation of the inhibitory effects of N6-cyclopentyladenosine on population spike amplitude was observed, resulting in a left-shift in the E-S curve. Intracellular recording indicated that N6-cyclopentyladenosine raised the threshold for spike induction by pulses of depolarising current, even at a concentration which did not produce hyperpolarisation of the neurone. At 30 nM, CGS 21680 prevented this effect of N6-cyclopentyladenosine, and this apparent antagonism was prevented by the A2A receptor antagonist ZM 241385. The results show that adenosine A1 receptors change the coupling between presynaptic transmitter release and postsynaptic cell firing, and that this effect is attenuated by A2A receptor activation.