SummaryLow concentrations (10-9 to 10-8M) of Cortisol strongly stimulate the proliferation of rat thymocytes maintained in vitro by a process which involves cyclic AMP. On the other hand, cortisol concentrations greater than 10-7M inhibit cell proliferation. The steroid's stimulatory effect may be due to sensitization of the cells to the mitogenic action of cyclic AMP.