(Plantain) fruit has been shown to be useful for nutritional, medicinal and industrial purposes. It contains serotonin (5-HT) and its immediate precursor, 5-Hydroxytryptophan (5-HTP). Serotonin plays a fundamental role in integration of behaviour and many physiological functions including regulation of mood, anxiety, arousal, aggression, impulse control, and thinking abilities. In this study, the effect of plantain-containing mouse diet on brain serotonin mice was investigated in mice. Thirty adult Swiss mice were divided into three groups of ten each and fed normal rodent chow containing 0%, 50% and 100% unripe plantain. After thirty days, the brain levels of 5-HT and 5-HTP were measured using High Performance Liquid Chromatography (HPLC) technique. Results show that 5-HT level was significantly increased in the plantain-diet fed mice. Conversely, 5-HTP level was reduced in the 100% plantain fed mice when compared with the control.