Microinjection of L-glutamic acid into the basal midbrain of the toad Bufo paracnemis induced a series of responses linked to antipredator behavior such as flight, backward locomotion and defensive postures. Furthermore, alerting/ orientation occurred in 20% of the animals, a behavior which is probably important for the animal to achieve the above responses. Locomotion occurred in 18% of the toads. The existence of a mesencephalic premotor control for locomotion in these animals is discussed comparatively. All responses were accompanied by changes in arterial pressure, heart rate and intrabuccal pressure. In some animals which exhibited backward locomotion responses, defensive postures and alerting/orientation, bradycardia or cardiac arrest occurred, with no appreciable changes in arterial pressure. This fact is discussed on the basis of studies which have shown the occurrence of similar alterations in situations of fear in other vertebrates. Autonomic changes without other responses occurred in 16% of the animals. Flight behavior was usually observed as a consequence of stimulation of sites located in the anterodorsal tegmental nucleus and lateral portions of the midbrain tegmentum. In contrast, locomotion was obtained by stimulating basal sites of the tegmentum located in the anteroventral nucleus and in the reticular formation lateral to it. Sites related to the remaining behaviors were located more diffusely in the basal midbrain.