The interactions between SK&F 104078 and several selective α2-adrenoceptor agonists at pre- and postjunctional α2-adrenoceptors were investigated in order to assess the previously reported selectivity of SK&F 104078 for postjunctional α2-adrenoceptors and to determine whether or not SK&F 104078 could uncover subtypes of α2-adrenoceptors located prejunctionally as has also been suggested. The α2-adrenoceptor agonists, UK 14,304, xylazine, B-HT 933, B-HT 920, clonidine and M-7, produced concentration-dependent prejunctional α2-adrenoceptor-mediated inhibition of neurogenic responses in the guinea pig atrium and rat vas deferens and produced postjunctional α2-adrenoceptor-mediated contraction of the canine saphenous vein. The α2-adrenoceptor antagonist, rauwolscine, blocked all the agonists at both pre- and postjunctional α2-adrenoceptors without demonstrating preference for any agonist or any synaptic location of α2-adrenoceptors. In marked contrast, SK&F 104078 produced equivalent antagonism of all agonists in the canine saphenous vein but had no significant effect against the same agonists in the guinea pig atrium, suggesting a high degree of selectivity for postjunctional α2-adrenoceptors in these test systems, consistent with our previous observations. In the rat vas deferens, however, SK&F 104078 significantly antagonized the prejunctional α2-adrenoceptor-mediated effects of clonidine and M-7 but did not block the responses to UK 14,304, xylazine, B-HT 933 and B-HT 920. These results indicate that the prejunctional α2-adrenoceptor antagonist effects of SK&F 104078 are tissue and agonist dependent, and that there may be at least two subtypes of prejunctional α2-adrenoceptors that can be discriminated with SK&F 104078 but not with rauwolscine. Both subtypes of prejunctional α2-adrenoceptors may be present in the rat vas deferens, while only the SK&F-10407 8-insensitive subtype is present in the guinea pig atrium.