The Golgi apparatus and associated membrane structures (Novikoff's GERL) of rat anterior pituitary cells were examined by electron microscopy with cytochemical and three-dimensional techniques. The so-called Golgi lamellae were divided into an outer and inner part. The former showed a negative response to both acid phosphatase (AcPase) and thiamine pyrophosphatase (TPPase) activities, but strongly reduced osmium tetroxide. The inner part was characteristically positive in TPPase activity, but negative to AcPase reaction and osmification. GERL was a flat cisterna usually situated inside the Golgi lamellae, having a thicker membrane with bristle-like coating. GERL was positive to AcPase but not to TPPase reaction and osmification. Three dimensional observations indicated that GERL was sheet-like with a peripheral extension of tubular processes, whereas the inner part of Golgi lamellae was reticular in shape. Both the inner part of Golgi lamellae and GERL can produce secretory granules. The silver methenamine method for detection of polysaccharides and glycoproteins showed strong reaction in the secretory granules, lysosomes, rough ER, Golgi lamellae and GERL. As the contents of the inner part of Golgi lamellae and GERL were the same, they probably did not differ in function. GERL may thus belong to the Golgi apparatus.