The present review will concentrate on a discussion of recent investigations that demonstrated how the levels of 5-Hydroxytryptamine (serotonin; 5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in the locus coeruleus, the dorsal raphe nucleus, frontal cortex and the hippocampus are influenced by tryptophan alone, tryptophan plus ethanol, and in combination with WAY100635, 5-HT_1A antagonist, 8-OH-DPAT, 5-HT_1A agonist, or SB224289, 5-HT_1B antagonist. Clinical studies have reported lower levels of cerebrospinal fluid 5-HIAA in aggression, suicide, and impulsivity.  On the other hand, compounds that increase serotonergic neurotransmission (uptake inhibitors, releasers, direct agonists) have been shown to decrease food intake. Thus, it is possible that serotonergic system that controls tryptophan intake may regulate alcohol consumption. The use of in vivo microdialysis techniques clearly identifies, in a rodent model, that acutely-treated tryptophan plus ethanol results in focal alterations in extra-cellular levels of 5-HT and 5-HIAA within the locus coeruleus, the dorsal raphe nucleus, frontal cortex and the hippocampus. This study provides evidence to indicate that the 5-HIAA level in the locus coeruleus, the dorsal raphe nucleus, frontal cortex and the hippocampus after tryptophan plus ethanol is indicative of changes in the activity of serotonergic neurotransmission in these areas and the participation of 5-HIAA as a general phenomenon in behavioral activation.