RECENTLY it has been shown that the brain is sensitive to low doses of ionizing radiation1,2. Prolonged exposure to X-irradiation decreases lever-pressing by rats3. Conditional aversion to saccharin results from exposure of rats to γ-radiation4,5. Because radiation affects psychological tests based upon food or fluid intake, we studied the effects of X-irradiation on rats trained to press a lever for electrical stimulation of the brain6. A single electrode was permanently implanted in the posterior hypothalamus of 8 CFN rats. After a training period of 6 months, the rats received 600 r. acute whole-body X-irradiation. Radiation characteristics were : 250 kVp.; 15 m.amp.; focus-object distance 70 cm.; filters, 0.21 mm. copper inherent, 0.5 mm. copper parabolic and 1.0 mm. aluminium; half-value layer 1.92 mm. copper; 22.9 r./min. measured in air.