The novel iboga alkaloid congener 18-methoxycoronaridine (18-MC) is a putative anti-addictive agent that has been shown, in rats, to decrease the self-administration of morphine and other drugs of abuse. Previous work has established that 18-MC is a potent antagonist at alpha3beta4 nicotinic receptors. Because alpha3beta4 nicotinic receptors in the brain are preferentially located in the medial habenula and the interpeduncular nucleus, the present study was conducted to determine if 18-MC could act in these brain areas to modulate morphine self-administration in rats. Local administration of 18-MC into either the medial habenula or the interpeduncular area decreased morphine self-administration while having no effect on responding for a non-drug reinforcer (sucrose). Similar results were produced by local administration into the same brain areas of two other alpha3beta4 nicotinic antagonists, mecamylamine and alpha-conotoxin AuIB. Local administration of 18-MC into the ventral tegmental area had no effect on morphine self-administration. These and other data are consistent with the hypothesis that 18-MC decreases morphine self-administration by blocking alpha3beta4 nicotinic receptors in the habenulo-interpeduncular pathway.