The present invention relates to a method for regulating the permeability of the blood brain barrier by administering a NOS-3 inhibitor to reduce the increased permeability of the blood brain barrier caused by a pathological condition or by administering a NOS-3 activator or nitric oxide donor to increase the permeability of the blood brain barrier. By increasing the permeability of the blood barrier, a therapeutic or diagnostic compound can be delivered across this barrier into the central nervous system.