1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) is an endogenous compound with neuroprotective and antidopaminergic activities. Our previous research has shown that 1MeTIQ prevents morphine addiction and abates the expression of the reinstatement of cocaine self-administration. The current study investigated the mechanism of action of 1MeTIQ that is responsible for its considerable anticraving potential. Accordingly, we performed behavioral tests that measured the influence of 1MeTIQ on the locomotor activity of rats (Wistar) after a single cocaine (15 mg/kg, i.p.) dose and during cocaine sensitization (15 mg/kg, i.p.). In a neurochemical study, we examined the influence of 1MeTIQ on dopamine release in the rat striatum after a single cocaine administration and during cocaine sensitization using an in vivo microdialysis methodology. The data showed that 1MeTIQ (50 mg/kg, i.p.) only slightly inhibited cocaine-induced hyperactivity but completely antagonized the expression of locomotor cocaine sensitization. The in vivo microdialysis study demonstrated that the administration of 1MeTIQ before the acute cocaine injection intensified the cocaine-induced increase in dopamine release and produced a huge and long-lasting elevation of the extraneuronal concentration of dopamine (by approximately 1400%, p < 0.01) in the rat striatum. A significant increase in 3-methoxytyramine (3-MT) (by approximately 400%, p < 0.01) was also observed. During the expression of cocaine sensitization, the administration of 1MeTIQ before the reminder dose of cocaine produced an additional elevation of dopamine release but considerably more strongly increased the concentration of 3-MT in the synaptic cleft (by about 800%, p < 0.01). In light of these data and of our earlier in vitro and in vivo experiments showing a physiological role of 3-MT in the inhibitory regulation of excessive stimulation, we suggest that locomotor hyperactivity is dependent not only on dopamine concentration in the extracellular space, but also on the ratio of [DA/3-MT]. 1MeTIQ administered before the reminder dose of cocaine to cocaine-experienced rats plainly normalized the [DA/3-MT] ratio, which was increased by cocaine, and this effect may be responsible for its anti-addictive action. The results strongly support the view that 1MeTIQ may have a more general anti-abuse potential, and the extraneuronal metabolite of dopamine, 3-MT, may play a crucial role in its anti-craving effects.