The effects of the alpha 2-adrenoceptor agonist clonidine and antagonist yohimbine were investigated on 3,4-diaminopyridine(DAP)-evoked [3H]norepinephrine ([3H]NE) release from rat hippocampal slices in the presence and absence of extracellular Ca2+. The slices were preincubated with [3H]NE and superfused in the presence of desipramine 1 mumol.L-1.[3H]overflow was evoked by addition of DAP 100 mumol.L-1 for 10 min to the superfusion medium. Clonidine and yohimbine inhibited and enhanced the 3,4-DAP-evoked [3H]NE release in a concentration-dependent manner both in the presence and absence of extracellular Ca2+. The effect of yohimbine was abolished by clonidine and was additive with the effect of ruthenium red. In the presence of extracellular Ca2+ the clonidine effect was not altered by addition of omega-conotoxin GVIA 0.1 mumol.L-1 or by removal of extracellular Ca2+, suggesting that the Ca2+ entry was not involved in the modulatory mechanisms of DAP-evoked [3H]NE release by activation of alpha 2-receptor. In the absence of extracellular Ca2+ the clonidine effect was reduced by the presence of ruthenium red 10 mumol.L-1, supporting the hypothesis that alpha 2-adrenoceptor activation might affect the intracellular mechanism of Ca2+ homeostasis.