Two groups of patients with the common complaint of excessive daytime sleepiness were examined. One group fulfilled the criteria for narcolepsy. The other had the sole symptom of excessive daytime sleepiness, confirmed by sleep latency tests, which was unaccompanied by systemic illness. Spinal fluid examinations for homovanillic acid, 3,4-dihydroxyphenylacetic acid, 3-methoxy-4-hydroxyphenylethylene glycol, and 5-hydroxyindoleacetic acid were carried out before and after administration of probenecid on the 20 patients and 40 controls. Probenecid-corrected accumulation of the dopamine metabolites was significantly increased in each of the patient groups, suggesting an association between pathological sleepiness and elevated dopamine turnover in the central nervous system.