In catfish hepatocytes (Ictalurus punctatus) α 1 -adrenergic activation by epinephrine or norepinephrine increased cytosol Ca 2 + concentration. This effect was inhibited by α 1 -adrenergic antagonists with the potency order WB4101 > benoxathian >= 5-methylurapidil > phentolamine > (+)niguldipine. Pretreatment with the irreversible antagonist, chloroethylclonidine, reduced the α 1 -adrenergic effect. α 1 -Adrenergic stimulation also increased the resynthesis of phosphatidylinositol in whole cells. In liver membranes, a relatively small (33 fmol/mg of protein) number of sites with high affinity (K d 100 pM) for the radioligand [ 1 2 5 I]HEAT was detected. Binding competition experiments showed the following orders of potency: (1) for agonists, oxymetazoline > epinephrine >= norepinephrine > methoxamine; (2) for antagonists, prazosin > WB4101 > benoxathian >= 5-methylurapidil > phentolamine > (+)niguldipine. Membrane pretreatment with chloroethylclonidine markedly reduced [ 1 2 5 I]HEAT binding. Good correlation was observed between the radioligand binding studies and the functional data with isolated cells. The present data suggest that the α 1 -adrenoceptor present in catfish liver cells belongs to the α 1 B subtype.