Urotensin II (UII) and its receptor GPR-14 have been identified in mammalian species, including man (Ames et al., 1999). Subsequently we and others have described profound renal effects of UII, including a marked reduction in renal blood flow and glomerular filtration rate (Song et al., 2003). UII and GPR-14 mRNA have been identified in human kidney (Matsushita et al., 2001) and UII immunoreactivity has been described along the human nephron, particularly in distal tubules and the glomeruli (Shenouda et al., 2002). Accordingly, the aim of this study was to identify immunoreactive UII and GRP-14 expression in the rat kidney and to measure urine UII concentration.Male Sprague Dawley rats (n = 11) were humanely killed and urine collected by bladder puncture. Kidneys either fixed and embedded in paraffin or minced for glomerular isolation. 4 µm sections were immunostained using specific antibodies for rat UII or GRP-14. Avidin-biotin-peroxidase (ABC) was used to reveal UII expression. Sections were developed in 3,3-diaminobenzidine and hydrogen peroxide. For [¹²⁵I]UII ligand binding, kidneys were decapsulated in ice-cold phosphate buffered saline, pH 7.4 (PBS). Cortical tissue was minced, washed through graded sieves with PBS, and centrifuged at 120g at 4°C for 5 mins. The pellet was resuspended in PBS, passed through a 23-gauge needle to remove Bowman’s capsule and re-centrifuged for a further 5 mins. The final yield was >80% glomeruli. UII was measured in urine by radioimmunoassay (Song et al., 2002).Strong immunostaining for both UII and GRP-14 was detected in the ducts of Bellini in the inner medulla, but was absent in the rest of the nephron. There was some specific binding of [¹²⁵I]UII to isolated glomeruli at the highest concentration used (1.2 x 10^-11 M), but no displacement by cold UII at lower concentrations. The concentration of UII in Sprague Dawley rat urine was 3.2 x 10^-9 M ± 4.7 x 10^-10 M (mean ± SEM, n = 5), which is 1642 fold higher than our previous measurements of UII in rat plasma (1.9 x 10^-12 M ± 4.9 x 10^-13 M, n = 5 (Song et al., 2002).These data show that UII and its receptor are expressed in the kidney of the rat, but this appears limited to the ducts of Bellini, contrasting with more widespread distribution in human kidneys. The low level of [¹²⁵I]UII binding in glomeruli suggests that the profound haemodynamic effects of UII on the rat kidney are mediated by pre-glomerular vessels. High UII urinary concentration suggests that UII may be secreted by the distal nephron.