TETANIC stimulation of high-threshold primary afferent fibers in the dorsal root was found to elicit intrinsic optical signals (IOSs) in transverse slices of 11 to 20-day-old rat spinal cords. The IOS, lasting for 30 s or longer, was most prominent in the lamina II of the dorsal horn. Treatment with a Na+-K+-2Cl−co-transport blocker, furosemide, abolished the IOS, suggesting that the origin of the IOS is the cellular swelling due to an activity-dependent rise in extracellular K+. Substance P antagonist spantide, glutamate antagonists 2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, and the μ-opioid agonist [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin suppressed IOSs. Thus, IOSs represent at least in part the slow excitatory response that is known to be generated in dorsal horn neurons after tetanic activation of unmyelinated afferent fibers.