Abstract: A subarachnoid pharmacodialysis apparatus insertable under the scalp, in and under the cranium, with a relatively short and simple neurosurgical procedure, to be kept there safely implanted for a year or longer for the purpose of regulating the neurochemistry of one or more diseased cerebral cortical areas and thus to achieve therapeutic effects via both localized delivery of medication and drainage of local neurotoxic molecules across the subdural meninges and compartments in a feedback-controlled fashion, with or without the additional capability of performing localized neurochemistry regulation in subcortical areas. This apparatus is also used for neurochemical profiling of the diseased brain area or areas by analyzing the removed endogenous molecules and adjusting the composition of the delivered medication based on the patient's specific, abnormal neurochemistry within the treated area or areas.