The release of [(3)H]d-aspartate from cultured cerebellar granule cells (a glutamatergic neuron) was studied. It was found that the l-glutamate induced stimulation of [(3)H]d-aspartate release from the neurons could be decreased 30-40% by adenosine, cyclohexyladenosine and (?)N(6)-(2-phenylisopropyl)-adenosine all in the dose range 1-10 ?M. The effects of adenosine agonists could be blocked and even reversed by the adenosine antagonist theophyllin indicating a possible endogenous adenosine tonus on the neurons. The results add further evidence to the notion that adenosine A(1) receptors present on parallel fiber terminals in the cerebellar cortex are involved in the regulation of glutamate transmitter release from the nerve terminals.