In order to study the three-dimensional architecture of the cytoskeletal system and its functional properties in the secretory cell, the anterior pituitaries of rats and mice were examined by the quick-freezing deep-etching method. The cytoplasm of the anterior pituitary cell is occupied by networks of several kinds of fine filaments. For convenience, the filaments which terminate at cytoorganelles, secretory granules, and plasma membrane were classified into three types. The filaments running away from the complicated networks of fine filaments terminate on the outer surface of the membrane of the cytoorganelles, secretory granules, and small vesicles and on the inner surface of the plasma membrane. These filaments, 4-10 nm in diameter, were termed associating filaments by us. There are direct connections by filaments between adjacent cytoorganelles, or between the cytoorganelle and secretory granule, or between the secretory granule and plasma membrane. These filaments are also 4-10 nm in diameter, and were termed connecting filaments by us. Short filaments, 3-8 nm in diameter, link directly at right angles the opposite membranes crossing over the cisterna of the nuclear envelope, of the rough endoplasmic reticulum, of the Golgi apparatus, and the intramitochondrial space and also the intercellular space. We named these linking filaments. These findings strongly suggest that these filaments are essential elements for supporting, maintaining and organizing the shape and location of all the cytoorganelles in the cell, and that they vary according to cellular function. The limiting membrane of the secretory granule is bound to the associating filaments and connecting filaments, running radially and at times connecting to the microtubules or plasma membrane. These filaments and microtubules may play a role in the organization and transport of secretory granules toward the plasma membrane.