Newborn rats were subcutaneously injected with 20 mg/kg body weight of hydrocortisone acetate each day for 5 days and were killed together with untreated controls 5 h or 10 days after the last injection. Histochemically demonstrable formaldehyde-induced fluoescence was studied in freeze-dried superior cervical ganglia and in strech preparations of the intestine and the aris. Fluorescence intensity was measured by visual estimation and microcope photometry. Hydrocortisone treatment caused a statiscally significant increase in the number of intensely fluorescent nerve cell bodies and a corresponding decrease in the number of moderately fluorescent nerve cell bodies in the superior cervical ganglia. In the Auerbach's plexus of hydrocortisone-treated rats all fluorescent fibers were more clearly delineated, showed more often varicosites and exhibited o more intense fluorescence, as compared with those of the control rats. The network of adrenergic nerve fibers in the iris showed, likewise, significant increase in the fluorescence fiber treatment with hydrocortisone. The increased nerve fiber fluorescence was observed also 10 days after the last hydrocortisone injection. It is concluded that hydrocortisone causes an increase in the content of noradrenaline in the sympathetic neurons, probably by increased synthesis.