This review highlights that essentially all of the recently discovered putative central nervous system (CNS) peptides and other peptide substances are measurable in human cerebrospinal fluid (CSF). Preliminary evidence also suggests that peptides in CSF may have an active regulatory role in relation to CNS function and behavior. Even if this is not the case, CSF peptides may prove to be a useful indirect marker of CNS peptide function and metabolism. Alterations in peptides have been reported in neurological and psychiatric illness, pain symptoms and their treatment, symptoms such as anxiety, and following treatment with CNS active drugs such as carbamazepine. CSF methodologies provide a strategy for the study of the interaction of classical neurotransmitters and peptide substances and their relationship to neural function and behavior in man. Assessment of peptides in CSF may supplement post mortem studies of peptide levels and receptor distribution and help lead to new diagnostic and treatment approaches in neuropsychiatric disorders.