The 5-HIAA/HVA ratio was determined in the cerebrospinal fluid in 5 patients with Parkinson's disease with L-DOPA preparations in which drug-induced dyskineses developed, in 5 patients treated with L-DOPA without dyskineses, and 10 controls. A decrease in the 5-HIAA/HVA ratio was observed in the group with dyskineses as compared with patients without dyskineses and in controls. The directions of further investigations on hte pathomechanism of drug-induced dyskineses are discussed.