Based on the previous studies reporting that BDNF and the 5-HT system are involved in various brain functions including anxiety, depression, aggression, learning and memory, and that dysregulations of these molecules are closely related to neuropsychiatric diseases [19–23], we hypothesized that these molecules are vulnerable to gravity changes and most likely mediate the effects of gravity changes on various types of behavior. This evidence concerns the gene BDNF and depressive disorder.