The loudness dependence of the auditory evoked potential (LDAEP) has been proposed as a valid biomarker of central serotoninergic activity in humans. The specificity and sensitivity of the LDAEP to changes in serotonergic neurotransmission have recently been explored in many studies about pharmacology and genetics. The majority of evidence for an association between the LDAEP and serotonin activity has come from animal studies. Genetic association studies with the LDAEP have provided conflicting reports with additional evidence outlining sensitivity to other neurotransmitter systems including the dopamine and glutamatergic systems. The LDAEP has been revealed to reflect the pathophysiology of various psychiatric illnesses. There is supporting evidence that major psychiatric disorders have differential LDAEP activities. Overall, the LDAEP shows strong evidence as a potential predictor of antidepressant treatment response. It need to be explored whether the LDAEP could be a biological marker of various psychiatric diseases and treatment prediction of antidepressants and serotonin related drugs.