Concentrations of major catabolites of dopamine were followed in the ventricular cerebrospinal fluid (CSF) in five patients undergoing intracranial pressure monitoring for chronic hydrocephalus. Determinations were made every 2 h following the administration of carbidopa/levodopa 25/250 mg (one Sinemet capsule) given 8h apart. The rise of homovanillic acid (HVA) concentrations was slow and progressive, reaching the level of statistical significance (p ≤ 0.01) only 8 h after the second administration of Sinemet. The rise in 3,4-dihydroxy-phenylacetic acid (DOPAC) was faster than the rise in HVA, with the peak value detected 4 h after the first administration of Sinemet. These data are interpreted as a confirmation, in humans, of a slow pool of exogenous levodopa, previously demonstrated in animal studies.