Investigation on mast cell tumor in aging mice in comparison with normal distribution of mast cell in rats. generally its well known that mast cell only occur in aging mice, no reference of mast cell tumor in rats. In this paper study of normal distribution of mast cell in rat with no evidence of tumor. these observation were done for several years. Mastomyosis was reported in human and the above studies will be very good for comparison of normal distribution of mast cell and incidence of tumor.