Changes in cerebrospinal fluid (CSF) levels of homovanillic acid (HVA), a major catabolite of dopamine and of 5-hydroxyindoleacetic acid (5-HIAA), the principle product of serotonin degradation, were compared in Guamanian parkinsonism-dementia (PD) and amyotrophic lateral sclerosis (ALS) patients with Guamanian and North American control subjects during the administration of probenecid. The rise of both HVA and 5-HIAA was significantly less in PD patients than in control subjects. Values for both these monoamine catabolites in patients with ALS consistently fell between those of PD patients and controls. Since probenecid inhibits the efflux of HVA and 5-HIAA from CSF, these results suggest that a substantial reduction in the synthesis of both dopamine and serotonin may attend PD and ALS of Guam.