Patients with Spanish Toxic Oil Syndrome (TOS) complaining of neuromuscular symptoms had increased levels of homovanillic acid and 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid. Severity of pain and muscle cramps correlated with the magnitude of increment in levels of monoamine metabolites. Mice treated with oleyl anilide, a putative toxic compound found in some stocks of the toxic rapeseed oil, did not present clinical or anatomical findings compatible with TOS. However, biochemical studies in these mice revealed a depletion of serotonin and an elevation of 5-HIAA levels. Our findings suggest that the unidentified toxic agent of the TOS and oleyl anilide induce pharmacological changes in monoamine neurons of the brain.