Homovanilic acid (HVA) was determined in the lumbar CSF of 12 patients with Huntington9s disease and 12 with Sydenham9s chorea before and after probenecid administration. The means of HVA concentration (basal and after probenecid) were lower in those with Huntington9s disease than in controls, and were even lower in a sub-group characterised by increased tone and slowness of voluntary movement. There was no correlation between CSF HVA values and the severity of abnormal movements, nor with length of the illness and age of the patients with Huntington9s disease. The mean basal HVA concentration did not differ from controls in those with Sydenham9s chorea but the accumulation with probenecid was significantly lower. These results suggest a decrease in cerebral dopamine release in both forms of chorea.