We studied the expression of c-fos in medullary serotonergic neurons after a locomotor task on a treadmill in adult mice. We used a transgenically modified line in which serotonergic cells expressed enhanced yellow fluorescent protein (eYFP). We counted and plotted cells using Micro Bright Field Co. software. We determined the location of the serotonergic cells in this mouse line in the adult. We found an increase in the number of eYFP-positive cells expressing c-fos after a locomotor task in the raphe and PPR between bregma -6.8 and bregma -6.48 in the mouse (flanked rostrally by the seventh cranial nerve and caudally by the inferior olive). The percentage of eYFP-positive cells that expressed c-fos after the locomotor task in the raphe was 4%, whereas in the PPR the percentage was13.3%. Our results corroborate the observation that a specific group of serotonergic neurons located in the PPR are involved in locomotion.