Electrical activity of the brain of Lithobius forficatus has been recorded. The addition of ecdysteroids (moulting hormone) in the haemolymph increased the frequency and amplitude of electrical response. Such a stimulation might be specific to these products since cholesterol and juvenile hormone had no action. The destruction or removal of various parts of the cephalic endocrine system demonstrated that the pars intercerebralis (p.i.) (postero-median area of the protocerebron) was responsible for increasing electrical activity after ecdysone supply.