The effects of noradrenaline (NA) on the inhibitory responses to GABA were studied in vivo in neurons of the cerebellar deep nuclei of the rat using extracellular recordings of single unit electrical activity and a microiontophoretic technique of drug application in loco. NA application influenced GABA-evoked inhibitions in ninety-seven percent of tested neurons. The effects of NA applications were depressive in the medial cerebellar nucleus (MCN) and in the posterior interpositus nucleus (PIN), enhancing in the anterior interpositus nucleus (AIN) and mixed in the lateral cerebellum nucleus (LCN). The mean intensities of NA-evoked effects were comparable in the various nuclei. The alpha(2) noradrenergic receptor agonist clonidine mimicked the action of NA on GABA responses in MCN and PIN (decrease) as well as in AIN and LCN (increase); this action was blocked by application of the specific alpha(2) antagonist yohimbine. The beta adrenergic agonist isoproterenol induced weak enhancing effects on GABA responses in all the nuclei with the exception of the LCN. Applications of both adrenergic agonist induced synergistic effects just in the AIN . These results confirm the hypothesis that the noradrenergic system controls the processing of cerebellar information and that this modulation is exerted in each nucleus in a different way by at least two mechanisms involving alpha2 and beta noradrenergic receptors.