We investigated the localization of mRNA encoding the calcium–sensing receptor (CaSR) along the rat nephron. For this purpose, we combined microdissection of nephron segments and RT–PCR techniques. The results indicate that mRNA encoding rat CaSR is present in rat glomeruli and distal segments (medullary thick ascending limb, cortical thick ascending limb, distal convoluted tubule and cortical collecting duct), whereas it was not detected in proximal convoluted tubules or proximal straight tubules. We also studied whether the CaSR transcription in kidney cortex was modified in response to low dietary phosphate. No significant changes were detected. Given the fact that a low–phosphate diet increased Ca<sup>2+</sup> excretion by more than 50–fold, the results suggest that if the CaSR regulates Ca<sup>2+</sup> reabsorption, it does so through receptor occupancy by Ca<sup>2+</sup> rather than by changes in receptor expression.