Major depression is one of the most common affective disorders caused by schizophrenia. The administration of N-methyl-D-aspartate receptor antagonists, such as ketamine, can reproduce the negative and affective symptoms of this disorder in animals. Preclinical studies have shown that 5-HT6 receptor (5-HT6R) agonists and antagonists have a considerable antipsychotic response. The aim of the present study was to evaluate the effect of an acute treatment with an agonist, E-6837, and an antagonist, SB-271046, of 5-HT6R on the immobility induced in mice by a subchronic ketamine regimen (5 days; 10 mg/kg/day, intraperitoneal). Repeated ketamine administration alone increased the immobility time in the forced-swimming test and the tail-suspension test. E-6837 at 10 and 20 mg/kg caused a significant reduction of immobility in the tail-suspension test and forced-swimming test, respectively. Interestingly, SB-271046 (10 mg/kg) also elicited an antidepressant-like effect in both tests. The current findings suggest an important role for these 5-HT6R ligands as mood modulators. However, it is necessary to explore the physiological mechanisms involved in this process in greater detail.