Since thyroid glycogen stores are low, the uptake of glucose is very important in order to maintain cell function (house-keeping). Previous studies have shown that TSH and insulin, independently, are regulators of this parameter. Since their corresponding mechanisms of action are different, we investigated the possible effect of the interaction between TSH and insulin on the stimulation of 2-deoxyglucose (2-DOG) uptake, a non metabolizable derivative of glucose. Confluent FRTL-5 cells were submitted to different treatments, usually for 72 h. In one series of experiments the concentration of TSH was kept constant, at 1 U/l, and the addition of insulin, from 0.16 to 1.6 micromol/l caused a progressive synergic increase in DOG uptake. When insulin concentration was kept constant, increasing amounts of TSH, from 0.5 to 10 U/l), also caused a synergic stimulation of DOG uptake. The effect of insulin was mimicked by IGF-1 (1-10 nmol/l), while that of TSH was mimicked by forskolin. Timecourse studies showed that TSH had a peak at 3 h of incubation, while insulin caused a progressive increase for up to 72 h. At short incubation times, up to 6 h, an additive effect of TSH and insulin was observed, while at longer times the interaction was synergic. The present results suggest that the interaction between the cAMP and the tyrosine kinase pathways on DOG uptake would involve two different mechanisms. At early times the effects of both hormones are additive, while in longer periods it becomes synergic.