Adrenocorticoid activity was investigated in experimentally induced protein malnutrition in Rhesus monkeys. Control studies were carried out in the same animals before inducing protein malnutrition. Plasma cortisol levels were elevated in the protein malnourished state. There was a total abolition of the diurnal rhythm of cortisol secretion. Fasting hypoglycemia was also observed in the protein malnourished state. It is concluded that increased adrenocorticoid activity and sustained steroidogenesis result from protein deficiency. Hypoglycemia may be an important stimulus, in addition to the metabolic stress imposed by the protein deprivation. The recognition of increased adrenocorticoid activity is important in a protein-deficient host, since the defense against infections might be impaired in such a situation.