LATS and TSH are chemically different thyroid-stimulating agents. To compare their early actions, we examined the effects in mice of LATS and TSH upon 2 thyroidal responses related to hormone secretion, namely, intracellular appearance of colloid droplets and release into the blood of preformed colloid-stored radio-iodine. Intracellular colloid droplets were induced by LATS in a manner qualitatively indistinguishable from the effect of TSH, but with a longer latency. In mice injected with 0.5 mU of TSH, a marked increase in the number of colloid droplets was observed in 10 min. and a maximum was reached in 30 min. A dose of LATS which was chosen to match 0.5 mU of TSH in its radioiodine-releasing effect (at 2 hr.) caused no increase in colloid droplets in 10 min., a small but significant increase in 30 min. and a maximal increase in 2 hr. When 16 times as much LATS was injected and shorter time intervals were examined, there was a slight increase in colloid droplets at 10 min., but the shape of the time-response curve was still grossly different from that of TSH, whose latency was greater than 4 but less than 7 min. Both LATS and TSH induced discernible release of thyroidal radioiodine only after the colloid droplet response was well established. Again, LATS showed a longer latency than TSH, and this difference in shape of the early part of the time-response curve could be used to distinguish the two agents. It is concluded that the early phase of stimulation of the thyroid by LATS is qualitatively similar to that due to TSH, but has a longer latency, which, among other explanations, might reflect a difference in the initial process of stimulation by the 2 materials.