Compounds of formula (Ia) as potent, .beta..sub.1-specific beta blockers with a short duration of action in the systemic circulation, wherein R is 3',4'-dimethoxyphenyl, R.sup.1 is hydrogen, and R.sup.2 is selected from methyl, ethyl, propyl, isobutyl and isopropyl; or R is 3',4'-dimethoxyphenyl, R.sup.1 is selected from fluorine, chlorine and bromine, and R.sup.2 is selected from methyl, ethyl, propyl, isopropyl, isobutyl and cyclopropylmethyl; of R is 4'-methoxyphenoxy, R.sup.1 is selected from hydrogen, fluorine, chlorine and bromine, and R.sup.2 is selected from methyl, ethyl, propyl, isopropyl, isobutyl and cyclopropylmethyl; or R is 3',4'-dimethoxyphenyl, R.sub.1 is cyano, and R.sup.2 is cyclopropylmethyl; or R is 4'-methoxyphenoxy, R.sup.1 is cyano, and R.sup.2 is isobutyl; and physiologically acceptable hydrolysable derivatives thereof having the hydroxy group in the 2-position of the 3-aminopropoxy side chain in esterified form, in their racemic and optically active forms, and their pharmaceutically acceptable acid addition salts.