Intact rat peritoneal mast cells were separated by elutriation into fractions differing in cell size, the smallest cells (less than 12 microns) exhibited high histamine synthetic activity (200-300 pmol of histidine decarboxylated per hr/10(6) cells) and contained little histamine (less than 2 pg/cell). With increasing cell size, histamine synthetic activity diminished to less than 20 pmol/hr/10(6) cells and histamine content increased to greater than 12 pg/mast cell. A gradation in histamine release in response to Compound 48/80 was also observed; small mast cells were resistant, those of intermediate size (12-13 microns diameter) were partially responsive and large mast cells (14-17 microns) were fully responsive to Compound 48/80. These changes were related to the state of maturity of the mast cells as indicated by their histochemical reactions to Alcian blue and safranin, which stain nonsulfated heparin precursors and heparin, respectively. Fractions of small mast cells were predominantly immature cells (70-85% of the cells were Stage I) with few granules, whereas fractions of large cells contained mostly mature cells (greater than 80% Stages III and IV) with numerous heparin-containing granules. Fractions in between contained mast cells at intermediate stages (Stages II and III) of development. Degenerative changes, namely decreased viability and diminished responsiveness to Compound 48/80, were evident, however, in fractions of the largest cells (greater than 17 microns). This and other work indicate that, even in adult rats, peritoneal mast cells are heterogeneous and can be separated on the basis of size into subpopulations that differ markedly in histamine content, histamine synthetic activity and ability to release histamine in response to Compound 48/80.