The alpha 1-adrenoceptors present in liver membranes from rats, hamsters and mice were characterized using [3H]prazosin. In the liver membranes from the three species a relatively large number of receptors was observed (500-900 fmol/mg of protein) and the affinities for [3H]prazosin were very similar (0.2-0.3 nM). Membrane preincubation with 10 microM chloroethylclonidine markedly decreased [3H]prazosin binding and higher concentrations essentially abolished specific binding of this radioligand. Binding competition experiments indicated the following orders of potency: a) for agonists: oxymetazoline > epinephrine > or = norepinephrine >> methoxamine and b) for antagonists: prazosin > WB 4101 > or = phentolamine = benoxathian > 5-methyl urapidil. The affinity for (+)niguldipine was also low but there was variation between the three species. Total RNA obtained from the liver of these species hybridized with the alpha 1B-adrenergic cDNA probe. The data suggest that these receptors correspond to the alpha 1B subtype.