Gastric ulcers were induced in rats by acetic acid treatment, and the mast cell kinetics in the lesions were studied. Within 24 h, mast cells had disappeared from the treated site and from the marginal zone, corresponding to the area of severe tissue injury. Regenerating epithelium appeared at day 10, and the lesion had healed by day 30. In this healing process, the number of mast cells was significantly increased, and their density in the regenerating mucosa, marginal mucosa, and marginal muscularis propria was 3.2 1.8, and 7.5-fold the control level, respectively. The increase in the number of mast cells was preceded by an increase in the percentage of S-phase mast cells. Mast cells in the mucosa were Alcian blue (AB)+/safranin (S)- and rat mast cell protease (RMCP) I-/II+, consistent with the features of mucosal mast cells throughout the observation period. On the other hand, most mast cells in the muscularis propria exhibited AB+/S+ and RMCP I+/II+ in the early period of ulcer healing. The latter feature was changed to RMCP I+/II- on day 30, indicating that immature CTMC appeared and then developed into mature CTMC during the ulcer healing. The significant change in the number of mast cells suggests that mast cells play an important role in the development and healing of gastric ulcers.