After reviewing briefly our earlier studies on glucocorticoid receptors and mechanisms in thymus cells, we have outlined results from the following two areas of current interest in our laboratories: the "life-cycle" of glucocorticoid receptors and complexes in thymus cells, and the levels of glucocorticoid receptors and sensitivity in immunologically stimulated human peripheral lymphocytes. Several of our results on energetics and kinetics of hormone binding to glucocorticoid receptors in rat thymus cells seem to require extension of the simplest model of hormone-receptor transformations in intact cells. ATP-depletion experiments suggest the existence of a nonbinding form of the receptor; "chase" experiments suggest reaction of hormone directly with nuclear-bound receptor; experiments on depletion and replenishment of cytoplasmic receptor using cortisol and dexamethasone suggest the existence of at least two subpopulations of nuclear-bound hormone-receptor complex. We have found that mitogen or immunologic stimulation of human peripheral lymphocytes in culture leads within 24 h or so to a striking increase in the number of glucocorticoid receptor sites per cell. We believe this increase may be due to partial synchronization of the cell population in a phase of the cell cycle in which receptor content is high. Contrary to the widely held view that mitogen-stimulated cells become insensitive to glucocorticoids, our experiments show that with respect to inhibition of thymidine and uridine incorporation and glucose uptake, the cells are highly sensitive to dexamethasone at 24, 48, and 72 h after stimulation with concanavalin A.