The renal kallikrein-kinin system has been implicated in the regulation of blood pressure and sodium/water excretion. The activity of renal kallikrein is controlled by a number of factors in vivo. Kallistatin is a newly identified serine proteinase inhibitor (serpin) which binds to tissue kallikrein and inhibits its enzymatic activity in vitro. To understand the role of kallistatin in modulating tissue kallikrein's function in vivo, we examined the anatomical relationship between human tissue kallikrein and kallistatin in the kidney by in situ hybridization histochemistry. Tissue kallikrein and kallistatin gene transcripts were identified using digoxigenin-labeled riboprobes at the cellular level. Antisense and sense riboprobes corresponding to the 3' region of the human kallikrein and kallistatin mRNAs were synthesized by in vitro transcription and used for hybridization. Using an antisense kallikrein riboprobe, sites of kallikrein synthesis were localized in the distal tubules, collecting ducts and Henle's loops of the kidney. To a lesser degree, juxtaglomerular cells were also stained. Kallistatin mRNA was found at the same sites where kallikrein mRNA was localized. The most intense signals of both kallikrein and kallistatin were seen in the distal tubules and collecting ducts. Hybridization was specific for the target mRNA since sense kallikrein or kallistatin riboprobe did not bind to the sections. Immunoreactive human renal kallikrein and kallistatin levels were measured in the kidney and urine by immunoassays using specific antibodies. Co-localization of kallikrein and kallistatin mRNA in the kidney suggests a potential role of kallistatin in regulating tissue kallikrein's function.