Sphingosine 1-phosphate (S1P) through its interaction with a family of G protein–coupled receptors (S1PR) is proving to have a significant impact on the activation of a variety of cell types, most notably those cells mediating the inflammatory response. Previously, we showed that S1P enhanced the excitability of small diameter sensory neurons, and mRNA for S1PR_1–4 was expressed in sensory neurons. These initial findings did not determine which S1PR subtype(s) mediated the increased excitability. Here, we report that exposure to the selective S1PR₁ agonist, SEW2871, produced a significant increase in excitability of some, but not all, sensory neurons. To further examine the role of S1PR₁, neurons were treated with siRNA targeted to S1PR₁. siRNA reduced S1PR₁ protein expression by 75% and blocked the sensitization produced by SEW2871, although some neurons remained responsive to subsequent exposure to S1P. Treatment with scramble siRNA did not alter S1PR₁ expression. Recordings from siRNA- and scramble-treated neurons suggested three distinct populations based on their sensitivities to SEW2871 and S1P. Approximately 50% of the neurons exhibited a significant increase in excitability after exposure to SEW2871 and subsequent S1P produced no additional increase; ∼25% were not affected by SEW2871 but S1P significantly increased excitability; and ∼25% of the neurons were not sensitized by either SEW2871 or S1P. RT-PCR measurements obtained from single neurons showed that 50% of the small diameter neurons expressed the mRNA for S1PR₁. These results indicate that S1PR₁ plays a prominent, although not exclusive, role in mediating the enhancement of excitability produced by S1P.