The naphthalene analog of medetomidine (1), 4-[1-(1-naphthyl)ethyl]-1H- imidazole (2), is a highly potent, selective alpha 2-adrenoceptor agonist. We have initiated a structure-activity relationship study of the replacement of the methyl group on the carbon bridge between the naphthalene and imidazole rings of 2 with a hydrogen, hydroxy, methoxy, carbonyl, or trifluoromethyl group and compared their biological activities with medetomidine 1 and the optical isomers of 2. Analogs of 2 were antagonists of alpha 2A-adrenoceptor-mediated human platelet aggregation and agonists on alpha 1- and alpha 2-adrenoceptors in guinea pig ileum. The rank order and potencies of these analogs on platelets (alpha 2A-subtype) and guinea pig ileum (alpha 1-subtype) were nearly the same, whereas racemic and S-(+)-2, desmethyl, and hydroxy analogs were potent agonists on alpha 2-adrenoceptors in guinea pig ileum. With the exception of the desmethyl analog 5, none of the other analogs were as potent as the parent drug 2 on alpha 2A- (human platelets), alpha 1- (guinea pig ileum), or alpha 2- (guinea pig ileum) adrenergic receptor systems. As with analog 2, the desmethyl- and methoxy-substituted analogs retained a greater alpha 2/alpha 1-selectivity in both functional (agonist activity) and biochemical (receptor displacement) studies. Receptor binding studies indicate that S-(+)-2 possessed greater affinity than the R-(-)-isomer on both alpha 1- and alpha 2-adrenoceptors in rat brain. In addition, R-(-)-2 did not show agonist activity in alpha 2-adrenoceptors of guinea pig ileum and was 10-fold more potent than S-(+)-2 as an antagonist of alpha 2A-adrenoceptors in human platelets. Thus, the nature of the substituent and the chirality at the carbon bridge between the naphthalene and imidazole rings play an important role in maintaining potent alpha 2-adrenoceptor activity and high alpha 2/alpha 1-selectivity within the 4-substituted imidazole class.