Pregnancy is associated with profound changes in renal haemodynamics and electrolyte handling. Relaxin, a hormone secreted by the corpus luteum, has been shown to induce pregnancy-like increases in renal blood flow and glomerular filtration rate (GFR) and alter osmoregulation in non-pregnant female and male rats. However, its effects on renal electrolyte handling are unknown. Accordingly, the influence of short (2 h) and long (7 days) term infusion of relaxin on renal function were determined in the male rat. Short term infusion of recombinant human relaxin (rhRLX) at 4 µg h^-1 100g bwt^-1 induced a significant increase in effective renal blood flow (ERBF) within 45 mins, which peaked at 2 h of infusion (vehicle, n = 6, 2.1 ± 0.4 vs rhRLX, n = 7, 8.1 ± 1.1 ml min^-1 100g bwt^-1 P < 0.01). GFR and urinary excretion of electrolytes were unaffected. After 7 days infusion of rhRLX at 4 µg h^-1, ERBF (1.4 ± 0.2 vs 2.5 ± 0.4 ml min^-1 100g bwt^-1 P < 0.05), urine flow rate (3.1 ± 0.3 vs 4.3 ± 0.4 µl min^-1 100g bwt^-1 P < 0.05) and urinary sodium excretion (0.8 ± 0.1 vs 1.2 ± 0.1 µmol min^-1 100g bwt^-1 P < 0.05) were significantly higher; plasma osmolality and sodium concentrations were lower in rhRLX-treated rats. These data show that long term relaxin infusion induces a natriuresis and diuresis in the male rat. The mechanisms involved are unclear, but do not involve changes in plasma aldosterone or atrial natriuretic peptide concentrations.