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Increased responses to intravenous injections of the full α2-adrenoceptor agonists, α-methyl noradrenaline and noradrenaline, were observed in conscious rabbits with perinephritis hypertension. However, responses to the partial agonist, BHT 920, and the antagonist, idazoxan, were unchanged. These increased responses to noradrenaline and α-methyl noradrenaline did not appear to be related to the α1 or β-adrenoceptor activity or to altered clearance of the agonists. The pro-aggregatory response of platelets from hypertensive rabbits and normotensive rabbits to adenosine diphosphate (ADP) was similar, but the sensitivity of platelets from hypertensive rabbits to the adrenaline potentiation of the pro-aggregatory response was increased. This was not related to changes in the number of α2-adrenoceptor binding sites or to changes at platelet β-adrenoceptors. In contrast the aggregatory responses to the partial agonist, guanabenz, were similar in normotensive and hypertensive animals.In rabbits with perinephritis hypertension responses to phenylethanolamines (full agonists at the α2-adrenoceptor) were increased in vivo in the conscious animal and in vitro in platelets, whereas responses to imadazolines which are partial agonists were similar in normotensive and hypertensive animals. These differences may be related to alterations in the coupling of α2-adrenoceptor activation to response in the hypertensive rabbits.