A useful method for the separation of labetalol into its two racemic diastereomers, as well as a stereoselective synthesis of its four stereoisomers, is described. The absolute stereochemistry of each isomer was determined by analysis of the DC spectra and confirmed by X-ray analysis. The alpha- and beta 1-adrenergic blocking properties, as well as the relative antihypertensive activities, have been measured in rats. The R,R isomer, 2a (SCH 19927), possesses virtually all of the beta 1-blocking activity elicited by labetalol and displays little alpha-blocking activity. In contrast, the S,R isomer, 3a, has most of the alpha-blocking activity. Of the four isomers, only 2a has antihypertensive potency comparable to that of labetalol. These findings, coupled with published data showing that labetalol possesses beta-adrenergic mediated peripheral vasodilating activity deriving essentially from its R,R isomer, lead to the following conclusion: The antihypertensive activity of labetalol can be ascribed to at least three identified complementary mechanisms, beta-adrenergic blockade, beta-adrenergic mediated vasodilatation, and alpha-adrenergic blockade, whereas the antihypertensive activity of 2a derives from the first two mechanisms only.