We used an in situ hybridization technique using single-stranded RNA probes to study the expression pattern of gamma-glutamyl transpeptidase (GGT) in rat liver and kidney during development. The results were compared to those obtained with an immunoperoxidase technique and with Northern blot analysis of GGT mRNA. In the kidney, northern blot revealed a 20-fold increase of GGT mRNA between day 18 of gestation and adulthood. Protein and mRNA localization clearly identified the proximal tubules as the site of synthesis of GGT. In the liver, the expression was lower than in the kidney and Northern blot showed a dramatic decrease of expression after birth. Using immunohistochemistry, the protein was detected within parenchymal cells in embryo and hepatocyte membranes and bile ducts in adults. Using in situ hybridization, GGT mRNA was only detected on days 1 and 2 after birth and exclusively in hepatocytes. Immunoperoxidase may be more sensitive than in situ hybridization to study the expression of minor liver protein such as GGT. However, the study of GGT expression using in situ hybridization is possible in cases of increased expression such as alcoholism, cholestasis, and carcinogenesis.