As the interactions of iodothyronines on adrenergic and vipergic receptors are not clear, the effect of exogenous T3 and T4 on catecholamine- and VIP-induced cAMP accumulation in human normal thyroid cells after eight days of primary culture has been investigated. To evaluate the effect of endogenous iodothyronines, the response of the adenylate cyclase system to isoprenaline, adrenaline, VIP, and TSH was studied during a 10 d period. T3 and T4 were unable to modify the catecholamine- and VIP-induced cAMP accumulation in human normal thyroid cells after 6-8 days of culture, while the response to TSH was significantly inhibited. In cells cultured from thyrotoxic tissue, the response of the adenylate cyclase system to catecholamines and VIP, during a 10 d primary culture, showed a behaviour similar to controls. TSH responsiveness was negligible up to the fourth day of culture, while in normal cells a response to all the agonists was present from the beginning. In view of the lack of effect of iodothyronines on catecholamine- and VIP-induced cAMP accumulation, and of the superimposable behaviour of the response to catecholamines and VIP in normal and hyperthyroid cells during the first days of culture, we can conclude that iodothyronines do not directly modify the response of the adenylate cyclase system to adrenergic and vipergic stimulation in human thyroid follicular cells. The lack of responsiveness to TSH of cells obtained from hyperthyroid tissue during the first 4 d of culture, associated with normal responsiveness to catecholamines and VIP, points to a possible involvement of biogenic amines and neuropeptides in sustaining such hyperthyroid states.