The present experiments were undertaken to study whether blockade of alpha 1-adrenoceptors would prevent the high-voltage spindle suppressing effect of atipamezole, an alpha 2-adrenoceptor antagonist. In adult rats, an alpha 1-adrenoceptor antagonist, prazosin, increased dose dependently the high voltage spindles in saline and atipamezole-pretreated rats. In addition, in aged rats prazosin blocked the high-voltage spindle suppressing action of atipamezole. Prazosin produced a smaller increase in high-voltage spindle values in aged than in adult rats. According to the present results, alpha 2-adrenoceptor antagonists may suppress high-voltage spindles indirectly by activating alpha 1-adrenoceptors in young and aged rats.