To determine if the adrenal gland plays a role in the thymic involution which occurs spontaneously in “lethargic” mutant mice, three different studies were made. Morphological studies were made first to determine if there was an indication of histological changes in the adrenal glands. Next, serum levels of corticosterone were measured by radioimmunoassay. Finally, mice were unilaterally adrenalectomized to see if such treatment would improve various symptoms of “lethargic” mutants. Results of the studies showed that lipid granules in the cortical cells of “lethargic” mutants were greatly reduced in number during the time of spontaneous thymic involution, and the mutant mice had a significantly higher level of serum corticosterone than the normal controls. Mutant mice unilaterally adrenalectomized at 15 days of age showed a marked improvement in their condition and their mortality rate decreased. It is concluded that spontaneous thymic involution of “lethargic” mutants is probably associated with adrenocortical hypersecretion.