1-Benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ), an endogenous neurotoxin was identified in mouse brain and cerebrospinal fluid (CSF) of normal human subjects, however in Parkinson’s disease (PD) patients its concentration in CSF is fortified [Kotake et al., J Neurochem, 1995]. Additionally, it was demonstrated in experimental studies that peripheral administration of 1BnTIQ causes a parkinsonism-like syndrome in rodents and primates. Recently, our experiments have shown that a single injection of 1BnTIQ produced a significant decrease in an exploratory locomotor activity, and a dramatic fall dopamine concentration in the brain. Interestingly, multiple 1BnTIQ treatments (50 mg/kg/day, ip × 10 days) resulted in development of tolerance to its dopamine depressing effect while the impairment of dopamine synthesis was persisted [W1sik et al., Neurotox Res, 2009]. It is well known that L-DOPA is the main medication used for the treatment of Parkinson’s disease, and in this study we evaluated the effects single and multiple 1BnTIQ (25 and 50 mg/kg/day, ip × 14 days) administration on L-DOPA (100 mg/kg, ip + Carbidopa 10 mg/kg, ip)-induced changes in motor activity and dopamine metabolism in the extrapyramidal brain structures (substantia nigra and striatum) of rat. The experiments were carried out on male Wistar rats weighing 250–280 g. The biochemical data were established by HPLC methodology with electrochemical detection. Results: the behavioral experiments have shown that both single and multiple administration of 1BnTIQ completely antagonized L-DOPA-induced an increase of horizontal and vertical locomotor activity in rats. Biochemical studies demonstrated that L-DOPA (100 mg/kg, ip + Carbidopa 10 mg/kg, ip) produced a strong increase of the rate of dopamine metabolism (by about 500%, p < 0.001), dopamine concentration (by about 200 to 300% of control level, p < 0.001), and a considerable much higher increase of the level its metabolites in striatum and substantia nigra. Both, acute and chronic administration of 1BnTIQ significantly antagonized all biochemical effects produced by L-DOPA injection. Conclusion: the present studies demonstrate that the stimulatory effect of L-DOPA on dopamine system was strongly impaired by 1BnTIQ. In connection therewith fact that 1BnTIQ is an endogenous substance present in the brain its concentration in PD patients may be a very important factor for efficacy of L-DOPA therapy in clinic.