Urotensin II (UII), a peptide hormone which influences glomerular filtration rate and urine concentration, and its receptor, UT, are expressed in the adult rat kidney. The ability of the kidney to reabsorb sodium and water starts to develop in utero and matures during early postnatal life in the rat, yet little is known about the ontogeny of the renal UII system. This study mapped renal expression of the urotensin system during the fetal and postnatal periods and determined renal activity of UII in the immature rat. Urotensin II peptide and mRNA were present in Sprague-Dawley (SD) rat metanephroi from the earliest stage examined, embyonic day 19 (E19; rat gestation 22 days); levels increased to peak at 4 weeks of age. In contrast, UT protein and mRNA expression declined rapidly between E19 and birth and remained at a similar level postnatally. Infusion of rat UII [6-60 pmol min-1 (100 g body weight)-1] or rat urotensin-related peptide [6 pmol min-1 (100 g body weight)-1] in anaesthetized 4-week-old SD rats had no influence on measured renal parameters; however, infusion of UT antagonist, SB-706375 (0.01 mg kg-1 min-1), provoked a pronounced diuresis [vehicle 23.5 ± 1.9 versus antagonist 75.3 ± 12.5 μl min-1 (100 g body weight)-1; P <0.001] and natriuresis, accompanied by modest increases in effective renal blood flow and glomerular filtration rate [vehicle 0.4 ± 0.1 versus antagonist 1.1 ± 0.2 ml min-1 (100 g body weight)-1; P <0.0001] and a significant increase in fractional sodium excretion. These results indicate that the endogenous rat UII system may influence renal sodium and water excretion before the onset of full urine concentrating capacity in the SD rat. © 2012 The Authors. Experimental Physiology © 2012 The Physiological Society.