The expression of the ornithine decarboxylase (ODC) gene in the mouse kidney following testosterone treatment was examined using in situ hybridization histochemistry. Testosterone (n = 5) or vehicle (n = 5) was subcutaneously injected (1 mg/animal) into male BALB/c mice (8 weeks in age) 14 h before sacrifice. Animals were sacrificed under ether anesthesia, their kidneys were removed and immediately frozen in liquid nitrogen. Frozen sections (10-microns-thick) were cut on a cryostat. Sections were hybridized with 35S-labeled sense or antisense RNA probe. The hybridization continued for 24 h at 50 degrees C and emulsion autoradiography was subsequently performed. A marked increase in ODC mRNA was exclusively detected in the proximal tubule of the renal cortex in the testosterone-treated animals. The hybridization signal was greater in the outer portion of the proximal tubule than in the inner portion. No significant hybridization signal was detected either in the distal tubule, renal corpuscle or peritubular tissues. These results indicate that testosterone induces the expression of the ODC gene in the proximal tubule of the renal cortex, leading to the increase in ODC activity in the same region.