Rats were exposed on gestational day 15 to 125 r X-irradiation, a treatment known to severely alter brain morphology. At 4-6 weeks of age the behavior of these animals was studied using two methods: (1) measurement of circadian locomotor activity, (2) photographic analysis of behavioral acts. The circadian locomotor activity of irradiated rats was similar to that of controls tested individually or as groups in a residential unit. After morphine sulfate, 2 mg/kg, the increase in locomotor activity was greater for the irradiated than control groups. Successive frames of photographic film were analyzed to determine the frequency, duration, sequencing of behavioral acts. No significant differences were present in these parameters of behavioral acts of control and irradiated rats. After morphine, irradiated rats showed a greater increase than controls in frequency and initiations of some behavioral acts, and these acts were more randomly dispersed in the sequences of acts. In this experiment activity was little affected by an agent which severely alters brain morphology, but the latent behavioral effects of the permanent brain damage became manifest, when the testing situation included challenge with a low dose of a drug which caused hyperactivity.