A series of 1-(substituted phenoxy)-3-(tert-butylamino)-2-propanols in which the ring substituents were 3,4-dihydroxy (6f), 3- and 4-hydroxy (6g and 6h, respectively), 3-hydroxy-4-methylsulfonamido (6i), its 3,4-transposed isomer (6j), and 4-methylsulfonylmethyl (6k) was prepared and examined for beta-adrenergic agonist and/or antagonist properties. Two of these compounds, 6f and 6j, were potent beta-adrenoreceptor agonists in in vitro tests that measure a compound's ability to relax guinea pig tracheal smooth muscle and to increase the rate of contraction of guinea pig right atria. Several compounds had a dose-dependent effect. Although they produced potent beta-adrenergic agonist activity at low concentrations, 6g, 6h, and 6j antagonized the effects of a standard beta-adrenoreceptor agonist at higher concentrations. The methylsulfonylmethyl derivative 6k produced beta-adrenergic blocking effects as demonstrated by attenuation of isoproterenol-induced increases in the rate of contraction of an isolated rabbit heart preparation. On the basis of these pharmacological results, coupled with NMR spectral data, it appears that the previous suggestion that aryloxypropanolamines interact with beta-adrenocreceptors as a consequence of their ability to assume an orientation in which the benzene ring the ethanolamine moieties can be superimposed on those of corresponding adrenergic phenylethanolamines is invalid. An alternative "bicyclic" rigid conformation involving two intramolecular hydrogen bonds in the protonated form of the aryloxypropanolamines is suggested to account for the similar beta-adrenoreceptor activity of these compounds and related phenylethanolamines.