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Quantitative and ultrastructural changes of mast cells (MC) were studied in rat lungs after inhalation of quartz particles. The number of degranulating MC was remarkably increased two weeks and 4 weeks after quartz inhalation (P < 0.05). The average number of the MC per unit area (mm2 of tissue section) and distribution in both experimental and control groups were similar and the mast cells were mainly distributed in the subpleural, perivascular and peribronchial connective tissues. The average number of MC per unit area significantly increased with pulmonary fibrosis three months later. Most of them infiltrated in the fibrotic alveolar septa and fibrotic areas, and the number of degranulating MC was also remarkably increased (P < 0.05). In addition, electronic microscopy showed that mast cells came into contact with fibroblast-like cells and the mast cells usually became degranulated in appearance. These results suggest that there is a close relationship between MC and silicotic fibrosis.