Stimulation of the hypothalamic defence area (HDA) in rats produces an increase of respiratory rate, tachycardia and a marked pressor response. The cardiovascular response is similar to that obtained during chemical stimulation of the A5 catecholaminergic region of the pons. We have demonstrated functional interactions between these two regions inhibiting A5 neurones with muscimol (Dawid-Milner et al. 2003). We aim to characterise the role of angiotensin II in the cardiorespiratory response to HDA stimulation. Experiments were carried out in spontaneously breathing anaesthetised rats (sodium pentobarbitone 60 mg kg-1 i.p., supplemented with 20 mg kg-1 i.v.; n=5). The response evoked by electrical stimulation of the HDA (1 ms pulses, 20-50 uA, given at 100 Hz, during 5 seconds) was analysed before and after the microinjection of angiotensin II into the A5 region (50 pmol, 50 nl, 5 s, in PBS pH 7,4 ± 0,1). All data were compared statistically (Student’s paired two-tail t test; results expressed as mean ± SEM). At the end of the experiments animals were humanely killed. Angiotensin II into the A5 region produced (10 minutes after the microinjection) an increase of mean arterial pressure (from 93.1 ± 2.64 to 113.2 ± 4.82 mmHg, p<0,001) and a decrease of respiratory rate (from 80 ± 1.37 to 72.2 ± 1.85 cpm, p<0,01). No changes were observed in heart rate (from 337 ± 8.6 to 341.8 ± 14.6 bpm). The compared cardiorespiratory response evoked by HDA stimulation before and after the microinjection of angiotensin II within the A5 region showed a decrease in the amplitude of the pressor response (from 45.2 ± 7.2 to 29.8 ± 8.1 mmHg p<0,05) and of the tachypnoea (from 41.8 ± 01.12 to 9.2 ± 1.62 cpm, p<0,05) together with the disappearance of the tachycardia (from 36.6 ± 4.2 to 2 ± 7.7 bpm, p<0.05). These results suggest a role for the A5 region angiotensin II receptors in the modulation of the cardiorespiratory response evoked from HDA stimulation. The abolishment of the heart rate component of the defence reaction suggests also its importance modulating the cardiac component the baroreceptor reflex.