To investigate the mechanism that monocytes participate in ischemic brain injury, endothelin (ET) levels in plasma, supernatants of cultured monocytes in vitro and those pretreated by dexamethason (DXM) were assayed by radioimmunoassay in 31 patients with acute cerebral infarction and 16 patients with hypertension (served as control). Compared with the control group, the ET levels in supernatants of cultured monocytes increased and positively related with those in plasma and degree of neurological damage in the acute cerebral infarction group, while the ET levels in supernatants of cultured monocytes pretreated by DXM decreased. This suggests that intensifying the synthesis and release ET may be one of the mechanisms for the involvement of monocytes in the pathologic process of ischemic brain injury during acute cerebral infarction. DXM can improve this process by inhibition of producing monocyte-derived ET.