Unilateral microinjections (0.20 microliter) of excitatory amino acids were made into the midbrain of freely moving rats. Injections made within the midbrain periaqueductal grey matter (PAG) consistently elicited reactions characteristic of defensive behaviour (i.e. explosive jumps, freezing, upright postures), whereas injections made in the tegmentum bordering the PAG did not as reliably elicit such behaviour. As injections of excitatory amino acids depolarize cell bodies but not axons, the results suggest that a population of neurones whose excitation elicits these reactions is found primarily within the midbrain PAG of the rat. Furthermore, the data suggested that such neurones may be localized preferentially within the caudal half of the midbrain PAG. Injections of the GABA antagonist, bicuculline methiodide, at many of the same midbrain sites produced behaviour similar to that elicited by excitatory amino acids indicating a possible GABAergic modulation of these same PAG-mediated reactions. It also was observed following unilateral injection into the PAG, of either the excitatory amino acid, L-aspartic acid or bicuculline, that defensive behaviour was elicited by touching the rat on the body or snout contralateral but not ipsilateral to the injection site. This suggests that the induction of defensive behaviour by unilateral PAG stimulation is due, at least in part, to lateralized alterations in sensorimotor responsiveness.