Background. Erythropoietin (EPO) has recently been shown to exert important cytoprotective and antiapoptotic effects in experimental brain injury and cisplatin-induced nephrotoxicity. The aim of the present study was to determine whether EPO administration is also renoprotective in both in vitro and in vivo models of ischaemic acute renal failure. Methods. Primary cultures of human proximal tubule cells (PTCs) were exposed to either vehicle or EPO (6.25-400 IU/ml) in the presence of hypoxia (1% O-2), normoxia (21% O-2) or hypoxia followed by normoxia for up to 24 h. The end-points evaluated included cell apoptosis (morphology and in situ end labelling [ISEL], viability [lactate dehydrogenase (LDH release)], cell proliferation [proliferating cell nuclear antigen (PCNA)] and DNA synthesis (thymidine incorporation). The effects of EPO pre-treatment (5000 U/kg) on renal morphology and function were also studied in rat models of unilateral and bilateral ischaemia-reperfusion (IR) injury. Results. In the in vitro model, hypoxia (1% O-2) induced a significant degree of PTC apoptosis, which was substantially reduced by co-incubation with EPO at 24 h (vehicle 2.5 +/- 0.5% vs 25 IU/ml EPO 1.8 +/- 10.4% vs 200 IU/ml EPO 0.9 +/- 0.2%, n = 9, P < 0.05). At high concentrations (400 IU/ml), EPO also stimulated thymidine incorporation in cells exposed to hypoxia with or without subsequent normoxia. LDH release was not significantly affected. In the unilateral IR model, EPO pre-treatment significantly attenuated outer medullary thick ascending limb (TAL) apoptosis (EPO 2.2 +/- 1.0% of cells vs vehicle 6.5 +/- 2.2%, P < 0.05, n = 5) and potentiated mitosis (EPO 1.1 +/- 0.3% vs vehicle 0.5 +/- 0.3%, respectively, P < 0.05) within 24 h. EPO-treated rats exhibited enhanced PCNA staining within the proximal straight tubule (6.9 +/- 0.7% vs vehicle 2.4 +/- 0.5% vs sham 0.3 +/- 0.2%, P < 0.05), proximal convoluted tubule (2.3 +/- 0.6% vs vehicle 1.1 +/- 0.3% vs sham 1.2 +/- 0.3%, P < 0.05) and TAL (4.7 +/- 0.9% vs vehicle 0.6 +/- 0.3% vs sham 0.3 0.2%, P < 0.05). The frequency of tubular profiles with luminal cast material was also reduced (32.0 +/- 1.6 vs vehicle 37.0 +/- 1.3%, P = 0.05). EPO-treated rats subjected to bilateral IR injury exhibited similar histological improvements to the unilateral IR injury model, as well as significantly lower peak plasma creatinine concentrations than their vehicle-treated controls (0.04 +/- 0.01 vs 0.21 +/- 0.08 mmol/l, respectively, P < 0.05). EPO had no effect on renal function in sham-operated controls. Conclusions. The results suggest that, in addition to its well-known erythropoietic effects, EPO inhibits apoptotic cell death, enhances tubular epithelial regeneration and promotes renal functional recovery in hypoxic or ischaemic acute renal injury.