Alpha 2-adrenoceptors were studied in platelets of stroke-prone spontaneously hypertensive rats (SpSHR) and of normotensive Wistar Kyoto control rats (WKY). In platelets of female SpSHR with established hypertension but not in those of normotensive WKY, specific binding of the alpha 2-adrenoceptor ligand, [3H]yohimbine, was found. Compared with human platelets (KD and B max about 3 nM and 200 fmol/mg protein, respectively), [3H]yohimbine binding to SpSHR platelets was of lower affinity (KD about 20 nM) and of lower capacity (B max about 30 fmol/mg protein). The potency orders of alpha-adrenoceptor agonists (clonidine greater than adrenaline greater than phenylephrine) and antagonists (yohimbine greater than or equal to phentolamine much greater than prazosin) in competing with [3H]yohimbine indicated that the binding sites in SpSHR platelets are of the alpha 2-adrenoceptors type. Similar data were obtained in platelets of 7-9 week old hypertensive rats without established hypertension. Furthermore, adrenaline inhibited SpSHR but not WKY platelet adenylate cyclase. This inhibition, which was smaller than in human platelets, was also found in platelets of 4 week old SpSHR but not in the corresponding control rats. The data show that in SpSHR alpha 2-adrenoceptors can appear, which may play a significant role in the pathogenesis of hypertension.