Genetic and other defects leading to brain changes in Down syndrome, Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, Gaucher disease, hypertension and other disorders are rapidly being identified. If brain access were possible, new candidates for gene replacement therapy, antisense oligonucleotides, immune proteins or growth factors might be used for treating these disease (Lowenstein et al., 1994; Wielbo et al., 1995). Further, a number of drugs, peptides, antibodies and biological response modifiers have proven valuable in inhibiting malignant, infectious and other pathological processes in vitro, but are unlikely to be employed clinically because of their limited access to brain.