An RT-PCR study of the distribution of connexin 43 (Cx43) mRNA in glomeruli and along the rat tubular segments was carried out to establish the differential expression of Cx43 in the different segments of the tubule, in renal regions, in isolated glomerular preparations (IGP), and in microdissected glomeruli. The mRNA level of Cx43 in macrodissected renal regions appeared in the following order: inner papilla > outer papilla and IGP > outer medulla and cortex. Among the microdissected tubules, inner medullary collecting ducts (IMCD) expressed the highest level of Cx43 mRNA, followed by the cortical collecting ducts (CCD). The proximal convoluted tubules and proximal straight tubules expressed significantly less Cx43 than the IMCD, glomeruli, and CCD. Medullary thick ascending limb and distal convoluted tubules showed the lowest level of Cx43 mRNA. The RT-PCR results of the microdissected segments correlate well with those obtained by RT-PCR of the renal regions. The high concentration of Cx43 mRNA in the IMCD together with the observation of abundant punctate immunofluorescence for Cx43 suggests that the IMCD not only expresses Cx43 mRNA but also that the mRNA is translated to Cx43 protein.