The Fos protein immunohistochemistry technique was used to map the brain areas activated by a 15-min exposure of rats to the elevated plus maze, an ethologically based animal model of anxiety. Two hours after the test, labeling was found mainly in the piriform and entorhinal cortices, amygdala, midline thalamic nuclei, several medial hypothalamic nuclei, periaqueductal gray matter, superior and inferior colliculus, cuneiform nucleus, dorsal raphe nucleus and locus coeruleus. These results support a participation of these structures in anxiety.