The kidneys play a pivotal role in the pathogenesis of essential hypertension because of a primary defect in renal hemodynamics and/or tubule hydro-saline handling that results in the retention of fluid and electrolytes. Previous studies have shown that increasing the renal pelvic pressure increased ipsilateral afferent renal nerve activity (ARNA), the ipsilateral renal pelvic release of substance P (SP) and the contralateral urinary sodium excretion in Wistar--Kyoto rats (WKy). However, spontaneously hypertensive rats (SHR) present an impaired renorenal reflex activity associated, partly, with a peripheral defect at the level of the sensory receptors in the renal pelvis. Furthermore, the renal pelvic administration of SP failed to increase ARNA in most of SHR at concentrations that produced marked increases in WKy. Since we have assessed the expression and localization of NK(1) receptor (NK(1)R), SP and calcitonin gene-related peptide (CGRP) in different dorsal root ganglia (DRG) cell subtypes and renal pelvis of 7- and 14-week-old SHR. The results of this study show increased SP and CGRP expression in the dorsal ganglia root cells of SHR compared to WKy rats. Additionally, there was a progressive, significant, age-dependent, decrease in NK(1)R expression on the membrane surface in SHR DRG cells and in the renal pelvis. In conclusion, the results of the present study suggest that the impaired activation of renal sensory neurons in SHR may be related to changes in the expression of neuropeptides and/or to a decreased presence of NK(1)R in DRG cells. Such abnormalities could contribute to the enhanced sodium retention and elevation of blood pressure seen in SHR.13835-44