Tissue kallikrein is a serine proteinase which processes kininogens to release bioactive kinins. Kinins mediate a variety of biological processes through the interaction with kinin receptors. Kinins are involved in the regulation of blood pressure and local blood flow, vasodilation, smooth muscle contraction and relaxation, production of pain and inflammation, and stimulation of cell proliferation. The tissue kallikrein-kinin system has been implicated in a number of pathophysiological processes such as hypertension, allergy and diabetes mellitus. In the present study, we have identified the expression and localization of components of the kallikrein-kinin system in the human eye by reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analyses, and in situ hybridization histochemistry. RT-PCR and Southern blot analyses have detected mRNAs of the key components of the system including tissue kallikrein, low molecular weight kininogen, and bradykinin B1 and B2 receptors at high levels in human retina, choroid and ciliary body, and relatively low levels in the optic nerve. In situ hybridization has identified cellular localization of these four mRNAs in ocular tissues. They are expressed in retinal neuronal cells including the outer nuclear layer, inner nuclear layer and ganglion cell layer. These mRNAs were also identified in endothelial cells of ocular blood vessels, ciliary muscle and lens epithelial cells. The sense riboprobes showed negative staining, which indicates the specificity of the antisense riboprobes. These results suggest that the tissue kallikrein-kinin system is produced endogenously in human ocular tissues. Similar expression patterns of kallikrein, kininogen and kinin receptors indicate that the kallikrein-kinin system may function in an autocrine or paracrine fashion in the eye.