Relationship between hypertension and monoamine oxidase (MAO) activity in rat brain and the change of this relationship by presynaptic ${alpha}-receptor$ agonist were studied. Animals were divided into three groups. Group I was composed of normotensive Sprague-Dawley rats (NR), group II of spontaneously hypertensive rats (SHR) and group III of acquired hypertensive rats induced by deoxycorticosterone acetate (DOCA) and NaCl treatment. Clonidine, a presynaptic ${alpha}-receptor$ agonist, was administered to groups II and III. Blood pressures and MAO activities were measured in each group. MAO activities in the brain of SHR were lower than those of NR. Animals in group II received clonidine which lowered blood pressures but did not change MAO activities in the brain. DOCA and NaCl induced hypertension 21 days after these treatments in group III and did not cause any changes in brain MAO activity. Clonidine lowered blood pressures of group III but did not change MAO activities. The data from the present study suggest that abnormaly low MAO activities in SHR brain may be one of the underlying factors for the susceptibility to hypertension and that the decrease in noradrenergic neuronal activities through presynaptic ${alpha}-receptor$ activation by clonidine may not be related to the changes of brain MAO activities.