In the present study, we hypothesized that acute diffuse brain injury (DBI) in rats would produce an increase in endothelin-1 (ET-1), a potent vasoconstrictor, and/or nitric oxide (NO), a potent vasodilator, in plasma and brain areas in rats. DBI was induced in anesthetized male Sprague-Dawley rats (350-400 g) using a 350 g weight dropped from 1 meter height impact through a device designed by Marmarou et al., 1994. Blood plasma and brain tissue (cerebral cortex, diencephalon and brain stem) samples were collected for estimation of ET-1 and NO at zero or 6 h from rats (n = 6) subjected to DBI as well as control rats (n = 6), i.e., not subjected to DBI. In a separate group of animals, cerebral blood flow (CBF) was recorded at 0, 5, 10, 15, 30, 60, 120, 240 and 360 min after induction of DBI or sham-DBI. Acute DBI produced a significant decrease in CBF at 120 min after induction of DBI. Plasma levels of ET-1 was found to be significantly increased (from 0.89 ± 0.09 to 2.09 ± 0.29 pg ml-1), at 6h following DBI. DBI produced a significant decrease in the levels of ET-1 in diencephalon (from 70.97 ± 9.47 to 57.64 ± 2.65 pg g-1). In contrast to ET-1, DBI produced a significant increase in the concentrations of NO in the diencephalon, cerebral cortex and brain stem at 6 h post DBI. It appears that DBI-induced increase in the levels of NO in brain regions which might be down regulating the synthesis of ET-1 in diencephalon. It is concluded that ET and NO homeostatic mechanisms may play a role in the regional and vascular responses associated with acute DBI.