We have studied the kinetics, on time scales of minutes and seconds, of formation and interconversion of glucocorticoid-receptor complexes in rat thymus cells under physiological conditions. Nonactivated and activated complexes were measured by a minicolumn technique that permits rapid, multiple simultaneous assays. The rate-limiting step in formation of nuclear complexes was activation, which at 37 degrees C had a half-time of 30-60 s. Activation in cells at 25 degrees C followed first order kinetics. Nuclear binding at 37 degrees C was too fast to measure, and probably has a half-time below 10 s. Earlier findings suggesting that triamcinolone acetonide and dexamethasone give higher steady state ratios of activated to nonactivated complexes than cortisol and corticosterone have been supported by showing that these ratios are concentration-independent, and are unlikely to be due to degradation or dissociation of complexes after cell disruption. A simple cyclic model of receptor kinetics, in which each glucocorticoid is characterized by its dissociation rate constant, accounts quantitatively for these results and many others. The model is based on the assumptions that activation is irreversible, and that energy is required for regenerating functional receptors after each cycle. It yields steady state ratios of activated to nonactivated complexes in agreement with experiment without introducing steroid-specific allosteric influences on activation, and suggests a new mechanism for explaining agonist-antagonist relationships.