Using dog liver membranes we observed that [125I]HEAT ((+/-)-beta-([125I]iodo-4-hydroxyphenyl)-ethyl-aminomethyl-tetralone) binds with high affinity (KD 97 pM) to a discrete number of sites (Bmax 40 fmol/mg protein) with the pharmacological characteristics expected for alpha 1-adrenoceptors. Such sites were inactivated by pretreatment with chloroethylclonidine. Binding competition experiments indicated the following order of potency: (a) for agonists: oxymetazoline > epinephrine > or = norepinephrine > methoxamine and (b) for antagonists: WB4101 > or = 5-methyl-urapidil = prazosin > or = benoxathian > or = (+)-niguldipine > phentolamine. Northern analysis indicated that total RNA isolated from dog liver hybridized with an alpha 1c selective probe (bovine brain). The orders of potency for agonists and antagonists, their Ki values and the Northern analysis suggest that dog liver expresses alpha 1A-adrenoceptors.