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Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made at sites within a restricted portion of the midbrain periaqueductal grey region (PAG) of the cat at which microinjection of the excitant amino acid, d,l-homocysteic acid, elicits the strongest form of a defence reaction, including a hypertensive response. Among the revealed projections, significant anterograde labelling was found in a discrete region of the rostral ventrolateral medulla, the subretrofacial nucleus (SRF). In the cat, the SRF contains pressor neurones which project to the spinal preganglionic sympathetic outflow. The labelling was most marked ipsilaterally, although substantial contralateral labelling was also observed. To verify that the projection to the SRF originated from the restricted ‘defence region’ of the PAG, WGA-HRP or rhodamine-labelled microspheres were injected into physiologically-identified sites in the SRF. In all …