1. We have compared prejunctional alpha 2-adrenoceptors in rat and guinea-pig vas deferens and rat and guinea-pig atria with postjunctional alpha 2-adrenoceptors in human saphenous vein and human platelets employing the antagonists yohimbine and SK&F 104078 and other alpha 2-adrenoceptor antagonists. 2. Yohimbine was approximately 10 times more potent prejunctionally than SK&F 104078 at antagonizing the inhibition by the alpha 2-adrenoceptor agonist xylazine of stimulation-evoked contractions in rat and guinea-pig vas deferens, and at increasing stimulation-evoked release of tritium in rat and guinea-pig atria pre-incubated with [3H]-noradrenaline. 3. Yohimbine was approximately 10 times more potent postjunctionally than SK&F 104078 at antagonizing contractions to noradrenaline in human saphenous vein and at displacing [3H]-yohimbine binding in human platelet membranes. 4. For the antagonists yohimbine, SK&F 104078, prazosin, phentolamine, CH 38083 and urapidil, there was a significant correlation between prejunctional potency in rat vas deferens atrium and postjunctional potency in human platelet, although the correlation was improved by the omission of prazosin. 5. We have no evidence for differences between functional pre- and postjunctional alpha 2-adrenoceptors in the periphery, although these functional receptors may differ from the ligand binding site in the human platelet.