Levels of homovanillic (HVA), 5-hydroxyindoleacetic acid and probenecid in cerebrospinal fluid were estimated in 42 patients with Parkinson’s disease undergoing the probenecid test. Relationship of the levels of amine metabolites and the serverity of the illness and the improvement after L-dopa therapy were established. One group of patients was treated with L-dopa alone, while the other was treated with L-dopa in combination with a peripheral decarboxylase inhibitor. We found a very high correlation between the HVA levels and improvement after L-dopa therapy in patients with relatively high levels of the dopamine catabolite. Patients with normal HVA levels in the cerebrospinal fluid did not respond to L-dopa. Also patients with extremely low HVA levels did not respond to L-dopa in a predictable way. Cerebrospinal fluid levels of 5-hydroxyindoleacetic acid was related to the pretreatment severity of illness. The present results suggest that the effec-tivity of L-dopa therapy is related in part to the deficiency of the central dopamine-containing system, while 5-hydroxytryptamine is also involved in Parkinson’s disease. With the probenecid test it is possible to predict L-dopa therapy failure only in those patients with relatively high HVA levels in lumbar cerebrospinal fluid.