The stability of the glucocorticoid receptor population in thymus cytosol prepared in a molybdate containing buffer was examined. Stability was initially assessed by an association time course. Those experiments demonstrated a slow decline in receptor specific binding, beginning at about eight hours of incubation at 4 degrees C. Stability was further assessed with a forward and reverse isotherm. The presence of hysteresis when the forward (association) and reverse (dissociation) isotherms were compared demonstrates that some element of the system either does not reach or does not maintain a stable state. A comparison with studies in muscle indicates that either the receptors or some component(s) of the cell free systems from thymus and muscle are different. Furthermore, equilibrium binding characteristics examined in thymus cytosols prepared in a molybdate buffer may be questionable due to instability in the receptor population.