Erythropoietin (EPO) promotes neuronal survival after cerebral ischemia in vivo and after hypoxia in vitro. However, the mechanisms underlying the protective effects of EPO on ischemic/hypoxic neurons are not fully understood. The present in vitro experiments showed that EPO attenuated neuronal damage caused by chemical hypoxia at lower extracellular concentrations (10(-4)-10(-2) U/ml) than were previously considered. Moreover, EPO at a concentration of 10(-3) U/ml up-regulated Bcl-x(L) mRNA and protein expressions in cultured neurons. Subsequent in vivo study focused on whether EPO rescued hippocampal CA1 neurons from lethal ischemic damage and up-regulated the expressions of Bcl-x(L) mRNA and protein in the hippocampal CA1 field of ischemic gerbils. EPO was infused into the cerebroventricles of gerbils immediately after 3 min of ischemia for 28 days. Infusion of EPO at a dose of 5 U/day prevented the occurrence of ischemia-induced learning disability. Subsequent light microscopic examinations showed that pyramidal neurons in the hippocampal CA1 field were significantly more numerous in ischemic gerbils infused with EPO (5 U/day) than in those receiving vehicle infusion. The same dose of EPO infusion caused significantly more intense expressions of Bcl-x(L) mRNA and protein in the hippocampal CA1 field of ischemic gerbils than did vehicle infusion. These findings suggest that EPO prevents delayed neuronal death in the hippocampal CA1 field, possibly through up-regulation of Bcl-x(L), which is known to facilitate neuron survival. (C) 2002 Wiley-Liss, Inc.