Adrenomedullin (AM) is a peptide hormone widely distributed in the central nervous system. Our previous study showed that AM gene delivery immediately after middle cerebral artery occlusion (MCAO) protected against cerebral ischemia/reperfusion (I/R) injury by promoting glial cell survival and migration. In the present study, we investigated the effect of delayed AM peptide infusion on ischemic brain injury at 24 h after MCAO. AM infusion significantly reduced neurological deficit scores at days 2, 4, and 8 after cerebral I/R. AM reduced cerebral infarct size at 8 and 15 days after surgery as determined by quantitative analysis. Double staining showed that AM infusion reduced TUNEL-positive apoptotic cells in both neurons and glial cells, as well as reduced caspase-3 activity in the ischemic area of the brain. In addition, AM treatment increased capillary density in the ischemic region at 15 days after I/R injury. Parallel studies revealed that AM treatment enhanced the proliferation of cultured endothelial cells as measured by both (3)H-thymidine incorporation and in situ BrdU labeling. Both in vitro and in vivo AM effects were blocked by calcitonin gene-related peptide (8-37), an AM receptor antagonist. Moreover, AM's effects were associated with increased cerebral nitric oxide (NO) levels, as well as decreased NAD(P)H oxidase activities and superoxide anion production. These results indicate that a continuous supply of exogenous AM peptide protects against I/R injury by improving the survival of neuronal and glial cells, and promoting angiogenesis through elevated NO formation and suppression of oxidative stress.