Serotonin has been implicated in a variety of neuropsychiatric disorders. Serotonergic dysfunction is thought to be involved in the pathophysiology of the Gilles de la Tourette Syndrome (GTS). GTS is characterized by multiple vocal and motoric tics. Among selective, competitive 5-HT(3) receptor antagonists, ondansetron represents a promising drug in GTS treatment. In our study both serotonin receptor subunit genes, HTR3A and HTR3B, were examined for sequence variations in GTS patients. We have analyzed DNA samples from 49 patients by SSCP and dHPLC. In HTR3A, we detected five mutations and in HTR3B the analysis revealed six sequence variants. Statistical analysis rated all variants as probably non-disease-related polymorphisms. Yet a certain effect of the detected variants on the severity of the disease cannot be excluded.