SUMMARY: 1. The cytochemical localization and changes of various phosphatase activities were studied in rat thyroid and anterior pituitary glands in various phases of secretory activity. The procedures used, made possible light microscopic study of the Golgi apparatus, plasma membranes and sinusoids, lysosomes, endoplasmic reticulum, nuclei and nucleoli; reactive sites many of which up to the present were in the realm of ultrastructure and could only be visualized with very specialized techniques or the electron microscope. The enzymatic make up and changes, and changes in size, shape and distribution of these structures were also studied in the conditions described. 2. Nucleoside phosphatase techniques visualized sinusoids, but this staining did not vary with secretory activity. The alkaline phosphatase procedure did not stain sinusoids of the anterior pituitary. However, the sinusoidal staining of the thyroid with this technique paralleled secretory activity. 3. Thiamine pyrophosphatase, inosine diphosphatase and guanosine triphosphatase staining visualized the Golgi lamellae in the two glands and this staining paralleled secretory activity. The adenosine monophosphatase technique stained occasional Golgi lamellae. 4. The adenosine monophosphatase procedure stained the plasma membranes of the less active cells of the anterior pituitary and granular cytoplasmic particles (lysosomes) of more active pituitary and thyroid cells. This granular cytoplasmic staining was abolished by sodium fluoride and d-tartaric acid as was acid phosphatase staining. 5. The inosine diphosphatase and guanosine triphosphatase techniques also revealed some diffuse cytoplasmic and cell membrane staining which was thought to represent staining of endoplasmic reticulum. 6. The adenosine diphosphatase procedure invariably stained nuclei and nucleoli.