Except for the localisation of tissue kallikrein (TK) in human prolactin secreting adenomas, its apparent deficiency in Alzheimer's disease and presence in human cerebrospinal fluid (CSF), the regional distribution of TK in human brain has not been defined. In this study on human brains collected within 24 h of death, we report on the regional distribution of TK in 24 different brain areas. The presence of TK was determined by its amidase and kininogenase activities, and the cellular localisation by immunocytochemistry. The highest amidolytic activity using the substrate D-ValLeuArg-pNA was recorded with extracts of the choroid plexus and hypothalamus, whereas extracts of the cerebral cortex (5 areas), cerebellum (3 areas), brain stem and thalamus possessed moderate amidolytic activity which correlated with the kininogenase assay. Immunoreactive TK has thus far been visualised in neurones of the hypothalamus, thalamus and reticular areas of the brain stem, as well as in cells of the anterior pituitary and choroid plexus by light and confocal microscopy. The cellular distribution of TK in specific areas suggests a role for TK in the neurones and epithelial cells of the brain. The question whether the functional importance of TK may relate to a particular cell type remains to be elucidated.