Background: Previous work has implicated isolated, control human lung mast cell granules in RNA metabolism using multiple methods of high-magnification imaging based on different mechanistic principles. These methods have demonstrated ribosomes, RNA, U1snRNP and uridine in, around and attached to secretory granules. Methods: Here, we have extended these studies using ultrastructural autoradiography of radiolabeled uridine incorporation in degranulating and recovering mast cells. Results: We found that control cells incorporated uridine into granules, with values that decreased dramatically in conjunction with stimulated histamine secretion and granule extrusion, and that granule stores of tritiated uridine increased together with the reconstitution of secretory granules in recovering mast cells. Conclusion: These findings support a possible new role for secretory granules in RNA metabolism in mast cell biology.