AbstractThree mouse thymocyte subpopulations with differing sensitivity to dexamethasone were characterized by size and density. Distribution patterns were determined for thymocytes during normal development and during regeneration after treatment with dexamethasone. From these, the subpopulation proportions were calculated and cell population kinetics were evaluated. Medium‐sized low density cells (type III) were least sensitive to dexamethasone and showed a relatively slow regeneration rate. The small cells of intermediate density, “early cortical” cells (type II), exhibited the most rapid regeneration rate, while the growth rate of the small cells of high density (type I) was intermediate. Cells of intermediate and high density were relatively sensitive to dexamethasone. The ratio of the phytohemagglutinin to concanavalin A (Con A) response increased after dexamethasone treatment with the increase in the proportion of type III cells. Con A responses were attributed to type II and type III cells. A tentative multipathway model for intra‐thymic T cell development is presented. Large cells, not confined to a certain trajectory in the density gradient, may represent a pool of proliferating cells of the other thre