Mast cells constitute a heterogenous cell system. The specific type of mucosal mast cell (MMC) of the gut differs with respect to a number of properties from the classical connective tissue mast cell ( CTMC ) found in, e.g. skin, tongue and the peritoneal cavity. This report summarizes recent findings concerning turnover rates of amines and heparin in the two cell types. The elimination rate of radioactively prelabelled histamine, 5-hydroxytryptamine (5-HT) and heparin from peritoneal CTMC was compared with the elimination of radiolabelled histamine from tongue, where histamine is stored in CTMC and duodenum where it is stored in MMC. The elimination of histamine from peritoneal CTMC was slow (t 1/2 = 23 days) and did not differ significantly from that of 5-HT (t 1/2 = 25 days) and heparin (t 1/2 = 35 days) suggesting a low degree of secretory activity in the normal rat. The elimination rate of histamine from the tongue was also very slow. The specific radioactivity of histamine in duodenum was decreasing more rapidly. This was explained by a dilution of the radioactivity since the histamine content increased during the experimental period, and also by MMC death. The results are compatible with the assumption that CTMC and MMC are secretory cells, but with low activity until recruited by adequate, immunological or other stimuli.