Abstract
1. Electrical stimulation of a small area of the hypothalamus, lying about 2 mm dorsal to the lateral margin of the mammillary bodies, evoked a characteristic complex of effects in the anaesthetized rabbit. 2. The observed effects included apneusis followed by tachypnoea, increased arterial blood pressure, and vagally mediated bradycardia. Exophthalmost and pupillodilatation was commonly seen. The ears were erected and the vibrissae drawn forward. Somatic effects were weaker, but often included increased extensor tone, especially in the forelimbs. 3. The intensity of these effects was dependent upon the frequency of stimuli within the trains. Effects were weak or absent below 20 shocks/sec, and were near-maximal above 60 shocks/sec, when employing constant-current rectangular wave stimuli of 100-300 muA, 1 msec pulse duration and 5-9 sec train duration. 4. The effects were not dependent upon connexions with the cerebral cortex, but were mediated by pathways that projected ipsilaterally from each side of the hypothalamus through the mesencephalon. 5. The evidence suggests that this region of the rabbit hypothalamus is partly homologous with that hypothalamic area associated with the defence reaction in cat. However, the over-all responses to stimulation in the anaesthetized rabbit differed considerably from those described in the anaesthetized carnivore, suggesting that their integrated physiological responses to emotional threat are dissimilar.