Using whole-cell patch-clamp methods, we examined the hypothesis that serotonin [5-hydroxytryptamine (5-HT)] receptor activation enhances TRPV1 function in mouse colon sensory neurons in lumbosacral dorsal root ganglia, which were identified by retrograde labeling with DiI (1,1Ј-dioctadecyl-3,3,3Ј,3-tetramethlindocarbocyanine methanesulfonate) injected into multiple sites in the wall of the descending colon. 5-HT increased membrane excitability at a temperature below body temperature in response to thermal ramp stimuli in colon sensory neurons from wild-type mice, but not from TRPV1 knock-out mice. 5-HT significantly enhanced capsaicin-, heat-, and proton-evoked currents with an EC 50 value of 2.2 M. 5-HT (1 M) significantly increased capsaicin-evoked (100 nM) and proton-evoked (pH 5.5) currents 1.6-and 4.7-fold, respectively, and significantly decreased the threshold temperature for heat current activation from 42 to 38°C.