A simple, rapid, and efficient method is described for establishing dog thyroid cells in tissue culture. Thyroid cell yield from a small amount of tissue (1 g) is high and viability is excellent. The adenosine 3',5-cyclic monophosphate (cAMP) response to thyrotropin (TSH) was investigated in these thyroid cells. Peak cAMP values were achieved after 10-15 min of TSH stimulation (in the presence of 0.5 mM 3-isobutyl-1-methyl-xanthine, MIX), with a subsequent decline to about half the maximal value after approximately 4 hours. This decline in cAMP concentration was associated with the development of refractoriness to TSH stimulation. Half-maximal stimulation of thyroid cell cAMP content was observed at a TSH concentration of between 1 and 2 mU/ml. Maximal cAMP values achieved were approximately 30-fold greater than basal values in the presence of MIX. The threshold of sensitivity of the cAMP response to TSH was very low, with significant stimulation being observed at a TSH concentration of 5-10 muU/ml. As determined by double reciprocal plots, the net cAMP response to TSH appeared to represent a single function over the entire TSH concentration range tested.