The cytochemical technique for endogenous peroxidase and an immunocytochemical method with protein A-gold were used to investigate the formation of the secretory granule substructure in acinar cells of the gerbil parotid glands in normal conditions and after treatment with the ionophore monensin. In untreated animals and 90 min after treatment with monensin, peroxidase activity was seen in rER, in transport vesicles close to the condensing vacuoles, and in the dense core of condensing vacuoles and secretory granules, whereas the Golgi cisternae remain unreactive. At 45 or 60 min after stimulation with monensin, the condensing vacuoles and the immature granules in the trans Golgi area did not exhibit dense cores and their content appeared similar to the peripheral portion of the mature secretory granules. These observations suggest that, in Mongolian gerbil parotid acinar cells, some substances such as peroxidase may be transported from the rER directly to the condensing vacuoles.