It has been postulated that abnormal characteristics of central noradrenergic nervous system has been implicated in the development and maintenance of hypertension in several modes of experimental hypertension including spontaneously hypertensive rats (SHR). In the present study, we attempt to determine if abnormal characteristics of central noradrenergic nervous system in SHR is caused by genetic factors or hypertensive phenomena by evaluating the changes of central adrenoceptors after long-term treatment of clonidine. Animals were divided into three groups; (1) 14 week-old SHR; (2) age-matched normotensive Wistar rats (NW); (3) DOCA-Salt induced hypertensive rats (DS). Clonidine (100 ug/kg) or vehicle was injected intraperitonealy twice a day for 15 days. Changes of ${alpha}_1-$ and ${alpha}_2-receptor$ desities following clonidine treatment were determiend in frontal corte, medulla oblongata and hypothalamus using 3H-WB4101 and 3H-clonidine, respectively. Densities of ${alpha}_1$ and ${alpha}_2-receptors$ following clonidine treatment were not changed in frontal cortex and medulla oblongate of SHR as well as DS, but increased in frontal cortex of NW and decreased in medulla oblongata of NW. On the other hand, densities of ${alpha}_1-receptors$ were increased and densities of ${alpha}_2-receptors$ were not changed in hypothalamus of SHR but densities of ${alpha}_1-$ and ${alpha}_2-receptors$ were decreased in hypothalamus of DS as well as NW. These results suggest that such differences in frontal cortex and medulla oblongata of SHR may be results of hypertensive phenomena whereas those in hypothalamus may be relevant to genetic factors of SHR.