1. Intra-vertebral artery-administered angiotensin II acts at the area postrema to facilitate central sympathetic vasomotor activity. Recent evidence suggests a possible role of the opiate system in the mechanism of action of angiotensin II at the level of the brain stem.
2. In these experiments, we show that the morphine antagonist naloxone reduces significantly the magnitude of the pressor response to vertebral artery-infused angiotensin II.
3. Morphine, in contrast, doubled the peak of the vertebral response to identical doses of the peptide. Neither naloxone nor morphine affected the pressor responses to intravenously administered angiotensin II.
4. The data suggest that the endogenous opiate system in the medulla modulates the cardiovascular effects of angiotensin II at the level of the area postrema.