Relaxin, a pregnancy hormone, has anti-apoptotic and anti-inflammatory properties. The aim of this study was to determine the effects of relaxin on ischemia/reperfusion (IR)-induced acute kidney injury. Male rats underwent unilateral nephrectomy and contralateral renal IR (45 min of renal pedicle clamping). Rats were divided into three groups: 1) sham group, 2) IR group, and 3) IR-RLX group (rats treated with relaxin before ischemia). In this group, relaxin was infused at 500 ng/hr via subcutaneous osmotic minipump for 24 h beginning 2 h before renal ischemia. At 24 h after reperfusion, renal function was assessed and kidneys were removed for analysis. There was no significant difference in blood pressure among the three groups. IR increased plasma levels of creatinine and urea nitrogen, and relaxin provided protection against the increases in these two parameters. Relaxin significantly decreased plasma TNF-α levels and renal TNF receptor 1 mRNA expression, compared to the IR group. Semi-quantitative assessment of the histological lesions showed marked structural damage in IR rats compared with the IR-RLX rats. RLX significantly reduced apoptotic cell counts compared to the IR group. Overexpression of caspase-3 observed in the IR kidneys was reduced in the IR-RLX group. The results demonstrated that relaxin provided protection against IR-induced renal injury by reducing apoptosis and inflammation.