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Unilateral microinjections (0.20 μl) of the excitatory amino acids (EAA), l-aspartate (ASP), d,l-homocysteate (DLH) or kainate (KA) were made into the midbrain of freely moving cats. Injections of DLH (20 nmol) or ASP (200 nmol) made within the midbrain periaqueductal grey matter (PAG) consistently elicited a threat display characteristics of defensive behaviour (i.e., pupillary dilatation, piloerection, retraction of the ears, sideways backing, arching of the back, hissing, howling, growling), whereas injections of DLH or ASP made in the tegmentum bordering the PAG did not elicit such behaviour. Injections of KA (940 pmol) made within the PAG, but not the tegmentum, elicited not only a threat display but also directed attack (striking with unsheathed claws and biting). As injections of EAA depolarize cell bodies, but not axons, the results suggest that a population of neurones whose excitation elicits all of the behavioural …