In the present investigation, the alpha-adrenoceptor blocking effect of (imidazolinyl-2)-2-dihydro 2,3 benzofurane or S 9871 and its stereoisomers was studied. In the pithed rat (+/-) and (+) S 9871 competitively antagonized the pressor effects of azepexole and clonidine more effectively than those of cirazoline and phenylephrine. (-) S 9871 only blocked the pressor response of the alpha 1-agonists used: phenylephrine and cirazoline. (+/-) and (+) S 9871 antagonized the inhibitory effects of clonidine on the increase in heart rate produced by stimulation of the sympathetic efferent fibres of the thoracic spinal cord. (-) S 9871 was twenty times less potent on the decrease in heart rate induced by clonidine. On the vas deferens of the rat, (+/-) and (+) S 9871 appeared to be more potent than (-) S 9871 in antagonizing the inhibitory effects of clonidine on the twitch response produced by electrical stimulation. Therefore, (+/-) and (+) S 9871 appear to be more preferential for alpha 2-adrenoceptors than for alpha 1-adrenoceptors; in contrast (-) S 9871 appears to be selective for alpha 1-adrenoceptors. (+/-) and (+) S 9871 appears to be one of the most selective agents for blocking alpha 2-adrenoceptors.