<i>Padawer</i> has proposed that rat mast cells in the peritoneal cavity have an extended lifespan and under normal conditions exhibit little turnover of their secretory granules. We have examined several constituents of the granules as part of a study of the structure and function of these organelles. The previously described increase in volume of mast cells with age of the rats from which the cells are obtained is associated with an increase in the quantity of histamine, heparin, <i>β</i>-glucuronidase and mast cell protease. While the former 3 granule components increase in parallel with mast cell volume, the increase in protease between 3- and 6-month-old rats is in excess of cell volume and the other 3 granule component. Granules from rats 1 month or less in age tend to be more difficult to isolate with their surrounding membranes intact.