The blood-brain barrier (BBB) is a major obstacle to gene therapy for genetic and infectious diseases in the central nervous system (CNS). At present, invasive techniques such as direct intracerebral injection of viral vector are necessary for clinical applications. Our laboratory is interested in developing a noninvasive cell-based gene delivery system for the CNS that exploits the natural ability of monocytes to enter the brain. With this in mind, fluorescently labeled monocytes were inoculated into the carotid artery of live mice, and their ability to enter the CNS was quantified by microscopic analysis of brain tissue sections obtained at necropsy. Labeled monocytes were detected in hippocampus, thalamus and cortex of experimental mice, and cell migration was confirmed by immunohistochemical analysis of tissue sections using a monocyte/macrophage-specific monoclonal antibody. Cell entry into the CNS could be enhanced by prior treatment of mice with intravenous bradykinin or hypertonic mannitol, with optimum results being obtained when bradykinin (5 mg/kg) or hypertonic mannitol was delivered 20 min prior to infusion of monocytes. These results suggest that it may be possible to transiently manipulate BBB permeability in such a way to achieve efficient migration of monocytes in the brain. This opens the way for the future use of vector-transduced monocytes as a novel delivery system to achieve effective gene transfer into the CNS.