The effects of intraventricular alpha2-adrenoceptor agonist and antagonist, clonidine and rauwolscine, on changes of blood pressure induced by the rise of intracranial pressure were investigated in urethane-anesthetized rabbits. 2) The rise of ICP, induced by the infusion of saline into a balloon placed in the epidural space, was comparatively slow in the beginning of the infusion but became sharp as the infusion proceeded. Corresponding with the gradual increase of ICP, there was a slight decrease in BP. An abrupt rise of BP was observed when ICP showed a sharp increase. 3) Intraventricular rauwolscine 5(microgram) by itself did not affect BP. In these rauwolscine-treated rabbits the increase of both ICP and BP by the infusion was similar to that of the control animals. 4) The pretreatment with rauwolscine 50(microgram) did hardly affect BP, but this made the increase of ICP and BP by the infusion different from that of the control animals. The slight hypotensive response in the beginning of the infusion did not appear and the pressor response to the raised ICP was markedly facilitated. The volume of saline inused into the infusing balloon to cause the same increase of ICP as in the control animals was much smaller than in the control ones, and the magnitude of the maximal increase of BP was much greater. 5) The pretreatment with 500 microgram of intraventricular rauwolscine produced an increase of BP. In these animals the increase of both ICP and BP by the infusion seemed to be slightly facilitated than in the control animals. 6) Intraventricular clonidine 30(microgram) markedly decreased BP. In these clonidine-treated animals the slight hypotensive response in the beginning was more distinct than in the control animals, and the pressor response was hardly seen. 7) The hypotensive response to intraventricular clonidine 30(microgram) was weakened in the animals pretreated with intraventricular rauwolscine 500(microgram). In these animals the increase of both ICP and BP by the infusion appeared as in the control animals. 8) The above results suggest that the pressor response to the raised ICP in rabbits was inhibited under the condition of stimulation of central alpha2-adrenoceptors and facilitated under the condition of blockade of the receptors. It seems that the rise of blood pressure takes place when the activity of alpha2-adrenoceptors is impared by the increased pressure of the balloon placed in the epidural space.