An inducible nitric oxide synthase (iNOS) mRNA was found primarily in the outer medulla of normal rat kidney. Identification of the mRNA was based upon the specificity of the oligonucleotide primers used for PCR amplification, PCR-Southern blot analysis and the nucleic acid sequence of the cloned PCR product. In addition to the outer medulla, glomeruli prepared from normal rat kidney contained significant amounts of an iNOS mRNA. These results suggest that there may be tonic influences in the outer medulla of the normal rat kidney resulting in the 'steady-state' presence of an iNOS mRNA. Cortical tubules and the inner medulla were found to contain detectable but lesser amounts of the iNOS mRNA. The outer medulla was microdissected into proximal straight tubule (PST), medullary thick ascending limb (MTAL), medullary collecting duct (MCD) and vasa recta bundle (VRB). The iNOS mRNA was found primarily in the MTAL with minor amounts in the MCD and VRB of normal rat kidney. Animals were injected with lipopolysaccharide (LPS) and sacrificed 24 hours later. Treatment with LPS caused at least a 20-fold increase in the amount of iNOS mRNA in the liver or in macrophages isolated from the peritoneum. Endotoxin treatment led to over a 10-fold increase in iNOS mRNA content in glomeruli and the inner medulla. The iNOS mRNA level of the outer medulla was increased two- to threefold due to LPS treatment.