The behavior of individual mice in rectangular plastic cages (25 x 9 x 6 cm.) With one half of the cage lead shielded and the other half exposed to a 3 Mev X-ray beam was studied. With no beam on, occupancy of the test cage was evenly divided between the two halves, indicating an unbiased environment. At a dose rate of 47.5 r/min. response was more variable and there was a slight tendency to prefer the shielded side. This tendency became definite at 100 r/min. and at 170 and 200 r/min. choice was made earlier. It is believed that this avoidance is not due to the presence of ozone or other noxious radiation products, or to visual clues from fluorescing plastic cage material.