Rats exposed to the smell of a predator adopted the freezing posture indicative of anxiety. Correlatively, the concentration of cholecystokinin tetrapeptide (CCK-4) was increased in the olfactory bulb, frontal and central cortex, dorsal and ventral striatum, central amygdala and the nucleus of the solitary tract. The concentration of CCK-8 was increased only in the ventral striatum. Glutamate was increased in the cortex and the striatum and dopamine was increased in the cortex. Intraperitoneal injection of CCK-4 increased brain levels of CCK-4 and replictted, in part, the behavioural effect of the smell of the predator. Injection of a CCK-B-receptor antagonist had the opposite behavioural effect. The results support a role for CCK-4 in anxiety.