Intradermal capsaicin in humans causes pain, primary hyperalgesia, and secondary mechanical hyperalgesia and allodynia. Parallel changes occur in the responses of primate spinothalamic tract cells and in rat behavior. Neurotransmitters that trigger secondary mechanical hyperalgesia and allodynia include excitatory amino acids and substance P. Secondary mechanical allodynia is actively maintained by central mechanisms. Our group has investigated mechanisms of central sensitization of nociceptive neurons by examining the responses to intradermal injection of capsaicin. These experiments are pertinent to issues raised by coderre & katz (sect. 2).