Ultrastructural and enzyme-cytochemical studies were performed on the Golgi apparatus in secretory cells of the lateral prostate of normal adult rats using serial ultra-thin sections. In the trans-Golgi area, a unique membrane complex composed of tubular portions and cisternal portions showing a rigid appearance is found. This corresponds to the GERL (Novikoff 1964) or the trans-Golgi network (Griffiths and Simons 1986). From their structural similarities, the cisternal portion found in this study is considered to be the same structure as the plate-like cisterna reported by Inoue and Kurosumi (1989). As we reported previously (Kimura and Ichihara 1985), there are at least two types of acid phosphatase (AcPase) in secretory cells of the rat lateral prostate: one is located only in the structural components involved in their secretory functions and reacts readily with naphthol AS-BI phosphate; the other is a lysosomal type, which reacts well with beta-glycerophosphate. Lysosomal AcPase activity demonstrated by Gomori's method (1952) was found in a few middle- to trans-Golgi cisternae and in lysosomes. The AcPase detectable with Gomori's method and thiamine pyrophosphatase seemed to exist in part in the same cisterna on the trans side. With Robinson and Karnovsky's method (1983) for lysosomal AcPase, however, the reaction products were found only in lysosomes that were occasionally tubular in shape. On the other hand, any activity of AcPases tested could not be detected in the cisternal portion with the rigid appearance. Thus, in secretory cells of the adult rat lateral prostate in normal condition, it is considered that the cisternal portion of GERL, or the trans-Golgi network, has no relation to the processing and/or transport of AcPases.