Several lines of evidence suggest that taurine acts as an organic osmolyte in the kidney. We investigated the cellular and subcellular distribution of this amino acid in rat renal tubule cells. Semi- and ultrathin sections of plastic-embedded rat kidney were incubated with an antiserum against conjugated taurine, using peroxidase-antiperoxidase and immunogold procedures, respectively. Extensive control tests confirmed the selectivity of the antiserum. Our immunocytochemical preparations revealed a highly differentiated labeling pattern. Strong labeling (judged visually or by computer-aided calculation of gold particle densities) was found in collecting duct cells throughout cortex and medulla, in proximal straight tubule cells, and in cells of the descending thin limbs of Henle's loop. Intermediate gold particle densities occurred in proximal convoluted tubule cells and intercalated cells of the collecting ducts (the gold particle in the latter being 30% of that in the collecting duct cells). The distal convoluted tubules, and thick and thin ascending limbs were almost immunonegative. It cannot be excluded that the proportion of free taurine that is retained by the fixative varies somewhat among the different cell types. Yet the highly differentiated labeling pattern that was obtained suggests that taurine is heterogeneously distributed among different populations of tubule cells, and that its level varies substantially even among cells that are exposed to the same osmotic stress.