Measurements of volume of the anterior pituitary and diameter of pituitary cells and differential counts of acidophilic and basophilic cells were made under normal or expcrirnental condition of bilateral or unilateral adrenalectomy or bilateral thyroidectomy in late fetal and newborn rats. The pituitary volume showed a consistent rise from the twentieth day of pregnancy to the third day after birth, but not as rapidly relatively as did body weight. Just after birth, the cell diameter declined.
Acidophilic and basophilic cells increased in per cent and stained more intensely.
In fetuses, bilateral adrenalectomy caused an increase in both the "pituitary volume/ body weight" ratio and the cell diameter. Unilateral adrenalectomy caused a n increase in the cell diameter. Thyroidectomy showed an increase in both the cell diameter and the per cent of basophilic cells which were paxtially degranulated.
In newborns, bilateral adrenalectomy caused the death of all operated rats. Unilateral adrenalectomy caused no significant change in all categories observed. Thyroidectomy showed an increase in per cent of basophilic cells, but not to the extent observed in fetuses. The basophilic cells were somewhat degranulated.
The observations support the views that in the rat the pituitary-adrenal and pituitarythyroid systems begin to function before birth and that the functioning of both systems is slightly reduced just after birth.