Urotensin II (UII) has been implicated widely in cardiovascular disease. The mechanism(s) through which it contributes to elevated blood pressure is unknown, but its emerging role as a regulator of mammalian renal function suggests that the kidney might be involved. The aim of this study was to determine the effect of UII on renal function in the spontaneously hypertensive rat (SHR). UII infusion (6 pmol.min^-1.100g bwt^-1) in anaesthetised SHR and control Wistar-Kyoto (WKY) rats produced marked reductions in glomerular filtration rate (GFR WKY, n=7, -0.3±0.1 vs SHR, n=7, -0.6±0.1 ml.min^-1.100g bwt^-1 P=0.03), urine flow and sodium excretion rates, which were greater in SHR by comparison with WKY rats. WKY rats also showed an increase in fractional excretion of sodium (FE_Na +0.6±0.1%, P=0.02) in contrast to SHR in which no such change was observed (FE_Na -0.6±0.2%). Blockade of the UII receptor (UT), and thus endogenous UII activity, with urantide evoked an increase in GFR which was greater in SHR (+0.3±0.1) compared with WKY rats (+0.1±0.1 ml.min^-1.100g bwt^-1 P=0.04) and was accompanied by a diuresis and natriuresis. UII and UT mRNA expression were greater in the renal medulla than the cortex of both strains, however expression levels were up to 3-fold higher in SHR tissue. SHR are more sensitive than WKY to UII, which acts primarily to lower GFR thus favouring salt retention in this model of hypertension.