Ventricular fluid concentrations of homovanilic acid (HVA) and dopamine (DA) were measured in 20 patients with the various types of hydrocephalus (seven with congenital hydrocephalus, five with aqueduct obstruction, five with secondary hydrocephalus after subarachnoidal hemorrhage and three with postoperative hydrocephalus of head trauma or a brain tumor. Probenecid loading test was also performed. By estimating changes in HVA, two types of hydrocephalus were differentiated (responder and non-responder). HVA concentrations were high in cases with acute hydrocephalus and responded well to probenecid loading. No response was observed in other types of hydrocephalus. In cases of acute aqueduct obstruction, the preshunting value or HVA was high and did not respond to probenecid loading. This result suggests that there is an active transport system for the acid metabolites, particularly HVA in man, in the choroid plexus of the fourth ventricle. When compared with the acute stage of hydrocephalus, ventricular dopamine concentrations were lower in the chronic stage of congenital hydrocephalus, and hydrocephalus secondary to a subarachnoidal hemorrhage or brain tumor. These results show that HVA excretion into the cerebrospinal fluid is lower in such hydrocephalic patients and that in some cases, administration of L-DOPA could be reasonably effective in ameliorating clinical pictures.