The organization and fine structure of granular convoluted tubule cells (GCT) from male mouse submandibular glands have been examined in controls and in animals injected with adrenergic and cholinergic secretagogues. Control submandibular glands exhibited a single population of GCT cells with numerous homogeneous granules filling the apical two-thirds of the cytoplasm. A zone of transition cells, exhibiting characteristics of both GCT and striated duct cells, was found between the agranular intercalated duct and GCT segments. These transition cells possessed apical granules of variable size as well as prominent basal striations. Dramatic changes in the morphology of GCT cells followed administration of the alpha-adrenergic agent, phenylephrine. The extensive degranulation involved formation of "secretory pools" of fused granules and release of secretory material into the lumen. The appearance of numerous smooth vesicles near luminal membranes suggested extensive membrane retrieval. Intracellular membrane-limited aggregates of membrane fragments suggested that much of the retrieved membrane was destined for degradation. Rough endoplasmic reticulum was highly dilated but there was no indication of increased size or activity of the Golgi complex. Ultrastructural evidence indicated that the secretory responses to isoproterenol, a beta-adrenergic agent, and to pilocarpine, a cholinergic agent, were much more modest, but it is clear that some secretory response to these agents does occur. The other cell types of the duct and tubule system did not exhibit comparable morphological changes in response to the agents used.