The inhibitory affinity of dopexamine hydrochloride to postsynaptic adrenoceptors, cholinoceptors, 5-hydroxytryptamine and histamine receptors was studied in rabbit isolated aorta. Dopexamine (10(-7)-10(-5) M) antagonized competitively the contractions of rabbit aorta evoked by noradrenaline (pA2: 6.60). Neither cocaine plus corticosterone nor cocaine, corticosterone plus propranolol altered the inhibition (pA2: 6.77 and 6.63, respectively). The antagonism of dopexamine against noradrenaline-evoked contractions was the same after 1 and 4 hr of pretreatment with dopexamine. In the presence of cocaine plus corticosterone, dopexamine antagonized the contractions evoked by phenylephrine (pA2: 6.94). Removal of endothelium did not influence this antagonism (pA2: 7.06). Dopexamine (10(-7)-10(-5) M) did not antagonize the contractions of aorta evoked by histamine (3 x 10(-7)-6 x 10(-5) M) and by 5-hydroxytryptamine (3 x 10(-7)-3 x 10(-4) M). Dopexamine (10(-8 and 10(-7) M) did not alter the contractions of endothelium-free aorta evoked by carbachol. Dopexamine (10(-7)-10(-5) M) slightly enhanced the contractions of aorta evoked by potassium (10(-2)-5.5 x 10(-2) M). These results suggest that dopexamine is an alpha 1-adrenoceptor antagonist. Furthermore, dopexamine has no affinity to cholinoceptors, histamine and 5-hydroxytryptamine (5-HT2) receptors and is apparently not a calcium antagonist.