The novel hypothalamic peptides orexin-A and orexin-B are known to induce feeding behavior when administered intracerebroventricularly, but little is known about other physiological functions. The renal sympathetic nerves play important roles in the homeostasis of body fluids and the circulatory system. We examined the effects of intracerebroventricularly administered orexins on mean arterial pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), and plasma catecholamine in conscious rats. Orexin-A (0.3, 3. 0 nmol) provoked an increase in MAP (94.3 +/- 0.7 to 101.9 +/- 0.7 mmHg and 93.1 +/- 1.1 to 108.3 +/- 0.8 mmHg, respectively) and RSNA (28.0 +/- 7.0 and 57.9 +/- 12.3%, respectively). Similarly, orexin-B (0.3, 3.0 nmol) increased MAP (93.9 +/- 0.9 to 97.9 +/- 0.9 mmHg and 94.5 +/- 1.1 to 105.3 +/- 1.7 mmHg, respectively). Orexin-A and -B at 3.0 nmol also increased HR. In other conscious rats, a high dose of orexin-A and -B increased plasma norepinephrine. Plasma epinephrine only increased with a high dose of orexin-A. These results indicate that central orexins regulate sympathetic nerve activity and affect cardiovascular functions.