The role of serotonin (5-HT) in behaviour may be influenced by environmental variables. Using calcium imaging, Seo et al. showed that 5-HTergic neurons in the mouse dorsal raphe nucleus (DRN) are less active during movement in low- or medium-threat scenarios (in an open field test or cued reward or cued avoidance tasks) and more active during movement in high-threat situations (the tail-suspension test and during escape from a foot shock). Photostimulation of 5-HTergic DRN neurons reduced and promoted movement in low-threat and high-threat environments, respectively. Thus, DRN 5-HTergic neurons may switch effects on movement depending on threat.