We have reported that clinically relevant concentrations of valproic acid (VPA) up-regulate the G-protein-coupled melatonin MT1 receptor in rat C6 glioma cells. To determine whether this effect occurs in vivo, the effects of chronic VPA treatment on the expression of both melatonin receptor subtypes, MT1 and MT2, were examined in the rat brain. Reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR analyses revealed significant increases in MT1 and MT2 mRNA expression in the hippocampus, following VPA (4 mg/ml drinking water) treatment for 17 d. Increases in the mRNA and protein expression of the novel neurotrophic factors, conserved dopamine neurotrophic factor and mesencephalic astrocyte-derived neurotrophic factor, were detected in the hippocampus and/or striatum. In addition, significant changes in persephin, glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor mRNA expression were observed. The robust multi-fold induction of MT1 and MT2 receptors in the hippocampus suggests a role for the melatonergic system in the psychotropic effects of VPA.