We investigated the effect of TSH on calcium transport in mouse thyroid, as well as the influence of iodide thereupon. Thyroid lobes were incubated in Krebs-Ringer bicarbonate buffer containing [45Ca2+] and the time-dependent uptake of [45Ca2+] by the lobes was observed. In the presence of 0.5 mU/ml TSH, the uptake of [45Ca2+] was significantly depressed at early phases of incubation (from 20 to 40 min). Similarly, (Bu)2cAMP (DBC) depressed the [45Ca2+] uptake. The efflux of calcium was also studied by using thyroid lobes preloaded with [45Ca2+]. In the presence of 0.5 mU/ml TSH, [45Ca2+] release from the lobes was doubled in comparison with the control lobes incubated without TSH. DBC similarly accelerated [45Ca2+] release from the lobes. The acute administration of excess iodide to mice fed a low iodine diet inhibits the TSH-induced adenylate cyclase activation in thyroids. The acceleration of calcium efflux induced by TSH was completely abolished by the acute administration of excess iodide in thyroids obtained from animals fed a low iodine diet. Similarly, DBC-induced acceleration of calcium efflux was inhibited by pretreatment with excess iodide. However, the inhibitory effect of iodide on TSH- or DBC-induced acceleration of calcium efflux was not observed in thyroids obtained from mice fed a regular diet. Inhibition of TSH-induced acceleration of calcium efflux by iodide was diminished by treatment of mice with methimazole before iodide. These results suggest that 1) TSH accelerates calcium efflux from the thyroid as a result of accumulation of cAMP in the thyroid, and that 2) iodide inhibits calcium efflux by inhibition of TSH-induced adenylate cyclase activation and by inhibition of the mechanism(s) which is (are) activated by cAMP.