Thirty albino rats were assigned to 30 treatment combinations according to sex differences, brightness contrasts present in the shuttle boxes, and radiation treatments. The proportion of time spent in the darker of two shuttle-box compartments was recorded hourly for 20 consecutive hours. Rats exposed to low-intensity irradiation in either the black only or in both the black and gray compartments spent significantly less time in the black compartment than did non-irradiated control Ss. Radiation produced a significantly greater change in behavior in female rats than in males. Radiation resulted in significantly less time in the black where the brightness contrast between compartments was slight than where it was great. When the behavior of Ss which could escape from radiation by crossing into the lighter compartment was compared with the behavior of Ss which could not escape in this manner, no significant differences were observed. This finding is interpreted as indicating that both groups spent relatively less time in the black as a generalized motor reaction to physiologic changes resulting from the radiation. The fact that Ss placed in shuttle boxes which were black on each end "escaped" from radiation by spending a significant proportion of time in the nonexposed side is interpreted, however, as suggesting direct sensitivity to the noxious effects of incident radiation.