The effects of ischemia on mast cells in the rat heart were studied. After 5 hours morphological signs of mediator release: granule dissolution, were observed. After 4 weeks the number of mast cells was significantly increased in comparison with nonischemic. Furthermore, 4 weeks after ischemia there was an increased number of regenerating mast cells. An increase in the number of mast cells and morphological signs of granule synthesis in the ventricular or septal muscle of the sham operated controls indicates that the surgical procedure itself can affect the mast cells of the heart generally. Morphological signs of granule synthesis were observed within mast cells of all experimental groups, and the figure was highly increased after 4 weeks of ischemia.