SUMMARY In the rat, a single subcutaneous injection of glucocorticoids, given during a sharply delimited 'critical period' following sensitization by a single oral dose of dihydrotachysterol, produces a precipitous and massive deposition of calcium salts in the thymus. Under comparable circumstances similar results are obtained by exposure to the stress of forced restraint, presumably through the intermediary of endogenous glucocorticoids. It is assumed that calciphylactic challengers can be liberated from disintegrating thymic tissue and that there may be close relationships between calciphylaxis, metastatic calcification and dystrophic calcification.