?-Adrenoceptor antagonists boast a 50-year use for symptomatic control in numerous cardiovascular diseases. One might expect highly selective antagonists are available for the human ?-adrenoceptor subtype involved in these diseases, yet few truly ?1-selective molecules exist. To address this clinical need, we re-evaluated LK 204-545 (1),(1) a selective ?1-adrenoceptor antagonist, and discovered it possessed significant partial agonism. Removal of 1 s aromatic nitrile afforded 19, a ligand with similar ?1-adrenoceptor selectivity and partial agonism (log KD of -7.75 and -5.15 as an antagonist of functional ?1- and ?2-mediated responses, respectively, and 34 of the maximal response of isoprenaline (?1)). In vitro ?-adrenoceptor selectivity and partial agonism of 19 were mirrored in vivo. We designed analogues of 19 to improve affinity, selectivity, and partial agonism. Although partial agonism could not be fully attenuated, SAR suggests that an extended alkoxyalkoxy side chain, alongside substituents at the meta- or para-positions of the phenylurea, increases ligand affinity and ?1-selectivity.