Nine rats with self-stimulation electrodes at various loci from mid-hypothalamus to anterior commissure were tested at current levels below and above threshold. In five, response rates were decreased by physostigmine, 0.1 mg/kg; in 3 they were decreased by physostigmine, 0.2 mg/kg; and in one they were not changed by either dose level. In the animals responding to 0.1 mg/kg (the only ones tested with other drugs) the quaternary compounds, neostig-mine and methylatropine, were without effect; but the tertiary compound, atropine, blocked the effect of the physostigmine. There exists in the rat a cholinergic system which can inhibit intracranial self-stimulation in animals with electrodes at various loci. The effectiveness of this system is variable, the variability being partially related to the gross location of the stimulating electrode.