Gap junctions are channel-forming protein complexes between neighboured cells that allow intercellular communication via diffusion of ions, second messengers or metabolites. Gap junctions consist of two hemichannels, each built of six connexin (Cx) protein subunits. To date, the connexin family in rodents comprises of 20 members. Little is known about the expression and functions of connexins in the kidney. We have therefore determined the expression pattern of all connexins in the developing and in the adult mouse kidney. By RT-PCR and immunohistochemistry we found expression of Cx26, Cx30, Cx32, Cx37, Cx40, Cx43, Cx45 and Cx46 in characteristic time and site specific patterns. Cx37, Cx40, Cx43, Cx45 and Cx46 were restricted to the glomerular-vascular system. Among all connexins found in the kidney highest expression levels were measured for Cx26 and Cx32, which colocalized with megalin-expressing proximal tubular cells. The developmental expressions pattern of Cx26 and Cx32 were similar. They were first detected at embryonal day 16 in a small subset of tubular structures colocalizing with megalin. Around birth, all proximal tubules expressed Cx26 and Cx32. Since apart from Cx26 and Cx32 no other connexin proteins could be found in the proximal tubules, we infer that these two proteins form the gap junctions between proximal tubular cells, which have been demonstrated by electron microscopy before. Since the functional meaning of gap junctions in the proximal tubule is yet unknown, our data suggest that the use of Cx32- and Cx26-deficient mice may help to shed first light on the role of gap junctions in the development and function of the proximal tubule.