We developed a novel behavioral test to evaluate in vivo function of nociceptors, which detect endogenous nociceptive/pain molecules in the primary afferent sensory fibers. The test using mice is sensitive enough to evaluate extremely low doses of pain inducers (PNAS 1998; J. Pharmacol. Exp. Ther. 1999, 2001, 2003; Br. J. Pharmacol. 2000). Another advantage of the test system is that we can repeat treatment of nociceptive/pain molecules to get the same intensity of pain signal in the same mouse. Thus, the approach enabled us to dissect signal transduction mechanisms by using nociceptive/pain molecules with inhibitors. By combining this technique with immunohistochemical analysis, we also classified three different nociceptive fibers of primary afferent nerve fibers. We also found plasticity of these fibers under the inflammatory, diabetic, and nerve injury-induced neuropathic pain conditions by evaluating nociceptive responses in each fiber. This study elucidated a mechanism by which tactile signals activate pain fibers under the condition of diabetic and peripheral nerve injury, and identified abnormal signaling in patients with diabetic and peripheral nerve damage; we consider this significant in the field. In summary, we identified in vivo signal transduction mechanisms of nociceptive/pain molecules, anti-nociceptive drugs, and neuronal plasticity in peripheral fibers under disease conditions.