Corticosterone in adrenal glands and plasma was meas-ured by a fluorometric method in rats fed a Mg-deficient diet, in 2 control groups receiving the same diet supplemented with different quantities of Mg, and finally in animals fed regular laboratory chow. Eosinophils in blood were also counted. Corticosterone in the Mg-deficient group showed a statistically significant decrease that was already apparent after 15 hr. on the diet and persisted throughout the 20 days of the experiment. The control diet containing the smaller supplement of Mg was sufficient to maintain body weight gain and to prevent erythema and eosinophilia but corticosterone levels were decreased also in this group. The animals receiving the larger Mg supplement showed corticosterone levels similar to those of the rats fed laboratory chow. The reduction of corticosterone is a sensitive index of suboptimal Mg intake.