The synthesis and structure-activity relationships of a series of threo-1-(aryloxy)-3-(alkylamino)butan-2-ols are discussed. These compounds are less potent beta-adrenoreceptor antagonists than the corresponding 1-(aryloxy)-3-(alkylamino)propan-2-ols. The data presented indicate that, unlike the arylethanolamine series, substitution of an alkyl group on the carbon atom alpha to the amino function on