The synthesis of a number of ring-halogenated N-isopropyl- and N-tert-butyl-2-amino-1-(2-thienyl)ethanols has been carried out in order to ascertain the influence of chloro or bromo substitution at the thiophene moiety on their blocking beta-adrenoceptor activity. It was found that chloro- and bromo-substituted compounds exhibited a similar activity. Monohalo substitution at positions C4 or C5 of the thiophene ring resulted in comparable blocking potency, whereas C3 halo-substituted compounds were practically devoid of activity. The highest activity in these series was observed with compounds dihalogenated at C4 and C5, their effect on myocardial beta-receptors being comparable to that of propranolol.