Serotonin (5-HT) is implicated in decision-making, such as signaling prediction errors and cue values, as well as to the regulation of behavioral states of wakefulness and arousal, but the relationship between these two domains of function is not well understood. On one hand, serotonin activity is negatively associated with locomotor behavior, which has been seen as behavioral inhibition. On the other hand, serotonin transients are also associated with surprise or uncertainty, as well as novelty responses, including exploration-related facial movements (e.g., whisking, sniffing). Because there may be a negative correlation between exploration and locomotion, we wondered if one of the associations of these functions with serotonin is primary. To investigate this, in this study, we explored in detail how both locomotion and exploration-associated facial movements correlate with 5-HT activity. We recorded the bulk calcium activity of 5-HT neurons in dorsal raphe nucleus (DRN) using fiber photometry in head-fixed mice running freely on a wheel while simultaneously recording their facial movements via videography. We found that DRN 5-HT activity exhibits a highly nonlinear relationship with locomotion speed. Specifically, during sustained running, increases in speed were associated with decreases in DRN 5-HT activity. However, during periods of quiescence, brief increases in speed were associated with increases in DRN 5-HT activity. In contrast, we found that facial motion exhibited a positive correlation with DRN 5-HT activity in both quiescence and sustained running. Lastly, as expected, we observed a nonlinear correlation between face motion energy and running speed, mirroring the relationship between DRN 5-HT activity and running speed. These findings support the idea that serotonin is primarily linked to arousal via involvement in regulation of responses to surprise and novelty, specifically exhibited by facial movements, with locomotor inhibition as a secondary consequence.