Abstract: Using radiant heat tail flick method and radioactive ligand binding test, the relationship between clonidine analgesia and central calcium cation was studied in rats. The analgesia of clonidine (1 mg/kg, sc) was antagonized by CaCl₂(1μmol/rat, icv) and potentiated by EGTA (0.2 μmol/rat, icv), a calcium chelator. Though the pretreatment with verapamil (0.1 μmol/rat, icv) showed no significant influence on the analgesia of clonidine (1 mg/kg, sc), it partly reversed the antagonistic effect of calcium. Calcium exhibited no inhibiting effect on ³H-clonidine binding at the concentrations 1 × 10^-7～1 × 10^-3 mol/L. The results suggest that analgesia of clonidine is closely rrelated with the calcium level of the tissue around the ventricles of brain. We consider that clonidine analgesia involves in central calcium, we also postulate that calcium takes part in a physiologic—biochemical process which is shared by the activated receptors in the endogenous analgesia system.