Hypophyses of non-thyroidectomized and thyroidectomized winter Necturi were divided into rostral and caudal portions which were frozen-dried refrigerated, and injected into mice for bioassay according to the method of McKenzie' ('58). Each mouse received the homogenized equivalent of two pituitary portions, either rostral or caudal. The per cent increase in blood I131 which resulted was used as a gauge of thyrotrophic activity.

In the non-thyroidectomized Necturus, nearly two-thirds of the thyroid stimulating activity was in the rostral portion which consisted of the rostral two-thirds of the anterior zone tissue. The remaining TS activity was in the caudal portion which consisted of the caudal third of the anterior zone, the transitional zone and basophilic bed area. These bioassay results coincide with the distribution of basophile cells described previously as thyrotrophs on the basis of histological observations.

After thyroidectomy, caudal portions yielded 95% of the thyrotrophic activity. This observation is consistent, with some reservation, with the number and distribution of thyroidectomy cells that differentiate after thyroidectomy, taking into account the vacuolation and degranulation of these cells which occurs in rostro-caudal sequence.