The membrane potential of cultured porcine thyroid follicular cells depolarized by up to 20 mV from the resting value of about − 73 mV on exposure to β-adrenoceptor agonists. A similar response was induced by TSH or dibutyryl cyclic AMP. α-Adrenoceptor agonists were without effect. The receptor subtype was shown to be (at least predominantly) β2 by the order of potency for β-agonists (isoprenaline ≅ fenoterol⪢adrenaline >noradrenaline) and by the relative potency of selective β-antagonists (ICI 118,551 ⪢atenolol). The α-agonist phenylephrine had no effect on the TSH response but weakly inhibited the β-agonist response. Rather than a physiological antagonism between α- and β-adrenoceptor-mediated responses, this effect was shown to be due to the weak β-antagonist effect of phenylephrine since the α-antagonist phentolamine failed to potentiate the depolarizing response to the mixed agonist nor-adrenaline, and also failed to block the inhibitory action of phenylephrine on the β-agonist effect. Sensitivity to β-agonist was enhanced by omission of serum from the culture medium and reduced by exposure to β-agonists or a high concentration of TSH or dibutyryl cyclic AMP.