The morphological and cytochemical changes of the Golgi apparatus and GERL of rat tracheal serous cells were studied after in vivo secretory stimulation with pilocarpine. Discharge of PAS-positive secretory granules had started within 30 min and complete discharge was finished within 2 h after stimulation. Reaccumulation of secretory granules became evident at 6 h and the volume of granules was similar to that of unstimulated control cells at 8 h. At 6 h after pilocarpine administration, acid phosphatase (AcPase) activity was markedly increased in GERL and immature secretory granules as compared to uninjected controls and this phenomenon continued 12 h after stimulation. GERL appeared to increase in extent and numerous continuities with immature granules were observed. Thiamine pyrophosphatase (TPPase) activity, normally present in trans Golgi saccules was not altered during the secretory cycle. At 2 h after stimulation, tubular structures of GERL, which also react to AcPase, were seldom observed. At 6 h, these structures were more numerous than those of control cells. These results demonstrate that, in rat tracheal serous cells, pilocarpine induces alteration of only GERL, and reconstitution followed by resynthesis of secretory granules is performed without participation of the Golgi stack.