1. The hypothalamic defence reaction was assessed in spontaneously hypertensive rats (SHR) and normotensive control rats (NCR).
2. SHR exhibited an apparently true hyper-reactivity of central autonomic structures mediating emotional behaviour during mental stress present in early as well as in established hypertension.
3. This hyper-reactivity is thought, therefore, to be a genetically linked factor predisposing to hypertension in SHR, rather than a consequence of the hypertensive state.