Forskolin is a potent activator of adenylate cyclase in many tissues including the thyroid gland. We compared the effects of 10(-5) M forskolin and 100 mu units/ml TSH on the dynamics of T4 and T3 secretion from perfused dog thyroid lobes. Both agents induced pronounced increases in T4 and T3 secretion. The increase in secretion was significantly steeper during forskolin than during TSH stimulation. This may suggest that early processes such as TSH-receptor interaction and the subsequent activation of the catalytic unit of adenylate cyclase are of importance for the pattern of very gradual increase in hormone secretion during TSH stimulation of the thyroid. In other respects forskolin seems to induce absolute and relative secretion of T3 and T4 very similar to those obtained by cAMP and TSH.