Studies on factors modulating the binding of agonist ligands to A1 adenosine receptors in rat forebrain membranes revealed that the reduction of [3H]cyclohexyladenosine [( 3H]CHA) binding, observed after removing Mg2+ by pretreatment with ethylene-dinitrilo-tetraacetic acid (EDTA), was restored by the polyamine, spermine (1mM). Parallel electrophysiological experiments performed on rat hippocampal slices in Mg(2+)-free medium indicated that spermine also led to a recovery of the depressive effect of 1 microM adenosine on stimulus train-evoked neuronal Ca2+ influx. These observations suggest that the polyamine, spermine is, like Mg2+, able to control the physiological adenosine-mediated modulation of synaptic transmission by changing the affinity state of A1 receptors.