The dorsal horn of the spinal cord is a key relay in the transmission of sensory information to the brain. Furthermore, this circuitry of spinal-cord neurons, and hence the spinal processing of sensory information, is subject to a great deal of plasticity, both pharmacological and physiological, in persistent pain states. This chapter describes in detail the procedure by which the activity and pharmacological modulation of these dorsal-horn neurons can be recorded in vivo in anesthetized rats, allowing a comprehensive study of spinal sensory processing in an intact and integrated system. The chapter covers the surgical preparation of the animal for electrophysiological recording; isolating and recording the activity of a single dorsal-horn neuron; and identifying the type of dorsal-horn neuron recorded by characterizing the neuronal response to a variety of peripheral stimuli. The study of these neuronal responses in a variety of persistent pain states, such as carrageenan-induced inflammation and neuropathy induced by L5/L6 spinal nerve ligation, together with the study of their pharmacological modulation by locally or systemically administered drugs, is also described.