Microinjections (0.05 μl) of l-glutamic acid, l-aspartic acid and dl-homocysteic acid were made in the midbrain at the same sites from which a defence reaction had been elicited previously by electrical stimulation. It was found that injections made at sites within the midbrain periaqueductal grey region (PAG) consistently elicited the facio-vocal signs of the defence reaction, whereas injections made at sites in the tegmentum bordering the PAG did not. As injections of excitatory amino acids depolarize cell bodies but not axons, the results indicate that the midbrain PAG but not the adjacent tegmentum contains a population of neurones whose excitation elicits these reactions. Furthermore, the data suggested that such neurones may be located preferentially within the caudal half of the midbrain PAG.