We have studied the degranulation in rat peritoneal mast cells by atomic force microscopy (AFM). Although AFM has advantages close to electron microscopy (EM) in spatial resolution, visualization of surface topography in peritoneal mast cells has not been reported yet. In the present paper we have succeeded in visualizing the degranulation process of rat peritoneal mast cells by AFM. AFM images showed that secretory granules were about 1 microm in diameter and that they were densely packed in the cytoplasm surrounding the nucleus. After stimulation with compound 48/80 (5 microg/ml), the packed granules began to loosen through formation of irregular cracks, as if polymerization of actin filaments was changed; 3 min after stimulation with compound 48/80 at 37 degrees C, the packed granules were completely loosened and the nucleus was no longer covered by the granules. In addition, AFM images showed clear fringes at the edges of the spreading cells. The fringes were 30-50 nm in height and there existed many small matrixes which were secreted to the outside of the cell. The matrixes were about 80 nm in diameter and 40 nm in height. These kinds of secretory matrixes were observed here first by AFM.