A histologic study was made of 50 human fetal pituitartes of gestational ages 5-40 weeks with the aid of the aldehyde thionin-periodic acid Schiff-orange G stain. Though pituitary basophils became apparent at 9 weeks, it was only at 13 weeks'' gestation that some of them took up the aldehyde thionin stain. The appearance of the primitive thyrotroph at 13 weeks coincides with the onset of fetal thyroid function. The typical polyhedral thyrotroph (beta-2 cell) did not appear until about 28 weeks.