Relaxin causes renal hyperfiltration, natriuresis, and diuresis in male rats. This study determined the effect of recombinant human relaxin (rhRLX) on renal function in the female rat. After 7 days of rhRLX treatment (0.4 μg/h) urinary excretion of sodium and chloride were significantly lower in the rhRLX-treated group, but urine flow rate was not different from that in controls. Quantitative PCR showed that the RXFP1 receptor was expressed in both the renal cortex and medulla; immuno-localization revealed expression in the proximal tubule and inner medullary collecting ducts. These data show that relaxin elicits pregnancy-like changes in renal function in the female rat.