The present study was undertaken to investigate the effect of clonidine on the response of the dorsal horn cells to intra-arterially administered bradykinin (BK:40μg) and K⁺(4mg) in spinal cats and cats with intact spinal cord. The change in the activities of low threshold (LT), high threshold (HT) and wide dynamic range (WDR) cells induced by BK and K⁺ were determined before and after treatment of animals with clonidine. Also studied was mechanism of inhibitory action of clonidine on the responses of dorsal horn cells to the chemical algogenics. Number of WDR cell responded to intra-arterially administered BK and K⁺ was greater in spinal animals than in cats with intact spinal cord. Following administration of BK or K⁺ no change was observed in the activity of LT cell whereas activity of HT cell increased invariably. The increased response of HT cell to BK and K⁺ was markedly suppressed by clonidine. On the other hand, such inhibitory actions of clonidine were almost completely blocked by yohimbine. The majority of WDR cells were activated by K⁺ while response of WDR cells to BK was diverse (excitatory, inhibitory or mixed). These results indicate that clonidine inhibits responses of the dorsal horn cells not only to thermal or mechanical stimulations but also to chemical algogenics, and that the inhibitory action of clonidine is generally mediated through excitation of α₂-adrenoreceptors.