Actively sensitized rat mast cells were stimulated to secretion in vitro by means of an anaphylactic IgE antigen reaction or with polymyxin B. Release of 5-hydroxytryptamine (5-HT) and heparin from the cells and from individual granules, extruded or displaced from stimulated cells with micromanipulation, was quantitated cytofluorometrically. Several similarities in action between the two secretagogues were found. All extruded and some intracellular granules from stimulated cells lacked 5-HT, and no heparin was released from the granules. Heparin secretion from mast cells was due to granule extrusion alone. Further, the proportions between 5-HT and heparin released from mast cells indicated that amine release, too, is mainly, but not exclusively, associated with overt granule exocytosis. The results suggested that the first extruded granules are the richest in both 5-HT and heparin. Antigen-induced secretion was not as vigorous as that induced by polymyxin and, unlike the latter, followed after a short lag of about 20 sec.