To evaluate the early age-related changes in neuropeptides, we have measured the cerebrospinal fluid (CSF) levels of beta-endorphin and substance P in young patients over a range of ages. Specimens of CSF were obtained from 39 neurologically normal children, aged 1 month to 10 years of age, and in 9 adult controls. CSF levels of both neuropeptides were observed to peak during the first year of life, and showed a negative correlation with increasing age. A significant positive correlation was observed between the CSF level of beta-endorphin and that of substance P.