Urotensin II (UII) has been implicated in transepithelial sodium transport in fish [1] and in mammalian renal function [2]. We have shown that infusion of exogenous rat UII in the adult Sprague-Dawley (SD) rat, at a dose with minimal effect on the renal vasculature, decreased renal tubular sodium reabsorption [3]. As urinary concentrating ability develops over the first few post-natal weeks in the rat, the aim of this study was to determine the renal actions of UII in young SD rats. Inactin anaesthetised (10mg.100g bwt^-1, i.p.) 4-week-old male SD rats (n= 5 per group) were infused i.v. with 0.154M NaCl containing ³H inulin (0.3μCi h^-1) and para-aminohippuric acid (3mg h^-1) as markers of glomerular filtration rate (GFR) and effective renal blood flow (ERBF), respectively, at a rate of 40μl min^-1 for 2.5h. Animals then received either vehicle or rat UII at 0.6 or 6 pmol.min^-1.100g bwt^-1 for 1h. Urine was collected from a bladder catheter every 15 mins, two blood samples (0.4ml) were taken via a carotid artery catheter. Animals were killed humanely at the end of the experiment. Rat UII at either dose had no effect on mean arterial blood pressure (vehicle 92±5 vs rUII at 6pmol 108±7mmHg) or ERBF (Table 1). Compared with the vehicle-infused group, rUII at 6pmol induced an initial 30% reduction in GFR after 15 mins (vehicle 0.6±0.1 vs rUII 0.4±0.1 ml.min^-1.100g bwt^-1) which returned to control levels after 45 mins. Rat UII also induced a 50% reduction in urine flow (UV) and sodium excretion rates (U_NaV) which reached a nadir after 1h. This was accompanied by a trend towards lower fractional excretion of sodium (FE_Na, Table 1). No changes were observed at the lower rUII dose. These data show that rUII influenced renal excretion indirectly through a reduction in filtered load, and are suggestive of a direct effect on tubular sodium reabsorption by the nephron. Results are consistent with reported expression of the UII receptor (UT) in the thin ascending limb of the loop of Henle and the inner medullary collecting ducts [2]. Young SD rats appear to be less sensitive to rUII than adults, which respond to 0.6pmol rUII [3], suggesting that UT expression is not yet fully mature at 4 weeks.