Western collared lizards, Crotaphytus collaris, were tested in three experiments using electrical stimulation of the brain. In experiment 1, agonistic behavior (defensive, aggressive and escape) responses were elicited in free-moving unanesthetized lizards. In experiment 2, areas were localized from which gular extension, a common component of defensive and aggressive behavior, could be evoked in anesthetized animals. Experiment 3 was carried out to demonstrate that defensive and aggressive behavior could be elicited from the same stimulation sites both in anesthetized and unanesthetized lizards. Initially, gular extension was evoked while the animal was anesthetized and later the animal was tested while freely moving and unanesthetized. Based on a combined plot of the sites from which defensive and aggressive behavior was evoked in experiments 1--3, the higher threshold sites (51--750 muA) were in the dorsal ventricular ridge anterior, amygdaloid complex, septal and preoptic areas, hypothalamus, thalamus and adjacent to the nucleus profundus mesencephali (NPM) and reticular formation. Lower threshold sites (up to 50 muA) are found in the NPM and the reticular formation. Escape behavior can be evoked from stimulation sites within or adjacent to areas from which defensive and aggressive behavior can be elicited.