Mature mast cells, isolated from the rat peritoneal cavity, were placed into suspension culture, either as resting or after degranulation by exposure to compound 48/80, and were maintained for up to 63 hr. No mitotic cells were observed, and cell number was conserved. The culture conditions did not cause spontaneous degranulation and cell survival was better than 80%. However, with time in culture, an increasing percentage of cells acquired a vesiculated appearance, characterized by a Golgi area with distended cisternae, the accumulation of lysosomal or autophagic-like vesicles, and enlarged, irregular or fused secretory granules. In the degranulated group, about one-fourth of the cells recovered the morphological appearance of resting cells by 63 hr, indicating that they are capable of 'recycling'. A cell type with a unique morphology, characterized by a large central vacuole containing secretory product, an eccentric nucleus, and mature secretory granules at the cell periphery appeared in the stimulated group after 22 hr of culture. In may be a possible intermediate stage in the mast cell regranulation process, based on its occurrence exclusively in the stimulated group, the correlation between its distribution and the recovery of mast cells to the resting state, and the morphological resemblance of its granule contents to stages in granule maturation in differentiating embryonic mast cells.