Focal cerebral ischemia elicits an inflammatory response characterized by the infiltration and accumulation of leukocytes, as well as the secretion of inflammatory mediators (Clark et al., Brain Res. Bull., 35 (1994) 387-392; Garcia et al., Am. J. Pathol., 144 (1994) 188-199; Wang et al., J. Neurochem. 71 (1998) 1194-1204). Leukocytes eliminate microbial invaders and necrotizing tissue debris, and can also turn against surrounding healthy tissue and exacerbate tissue injury (Furie and Randolph, Am. J. Pathol., 146 (1995) 1287-1301; Kochanek and Hallenbeck, Stroke 23 (1992) 1367-1379). Inflammatory mediators are considered to play an important role in attracting and stimulating leukocytes (Weiss, N. Engl. J. Med., 320 (1989) 365-376). Monocyte chemoattractant protein-1 (MCP-1) functions as an inflammatory mediator, whose source and role in focal cerebral ischemia is worth studying. MCP-1, a potent chemoattractant factor, may play an important role in ischemia-induced inflammatory response. The aim of the present study is to determine the time course and cell type of MCP-1 protein expression after permanent focal ischemia in mice. ELISA and immunohistochemical staining were used to detect the expression of MCP-1 protein after 0 h, 2 h, 4 h, 12 h, 1 day, 2 days, 3 days, 5 days and 7 days of middle cerebral artery occlusion (n=3-5 in each group). Double-labeled fluorescent staining was used to examine the cellular localization of MCP-1. The results demonstrated that MCP-1 expression was mainly observed in the ischemic core after 12 h of middle cerebral artery occlusion, then gradually increased and extended to the ischemic perifocal area. MCP-1 expression peaked at 2 days and 3 days, and gradually decreased after 5 days of MCAO. Double-labeled immunostaining for MCP-1 and neuron specific enolase (NSE) or glial fibrillary acidic protein (GFAP) showed that MCP-1 positive neurons were observed as early as 12 h of ischemia, while MCP-1 positive astrocytes were observed after 2 days of ischemia. These results support the functional role of MCP-1 in ischemic brain injury and reveal a distinct temporal and spatial expression of MCP-1 in cells believed to be neurons and astrocytes.