In rat spinal cord slice repetitive electrical stimulation of the dorsal root at an intensity that activates C-fibers evokes a slow-to-develop and prolonged (30-50 sec) change in light transmittance (OIS_DR) in the superficial part of the ipsilateral dorsal horn (DH_s). Inhibition of astrocyte metabolism (by bath-applied 400 µM fluoroacetate and 200 µM glutamine; FAc+Gln) or interference with glial and neuronal K⁺ transport (by 100 µM 4-aminopyridine; 4-AP) leads to dissociation of the OIS_DR and the postsynaptic DH_s response to a single-pulse, constant-current dorsal root stimulus (P-PSP_DR). The OIS_DR decreases under FAc+Gln whereas the P-PSP_DR remains unaltered; under 4-AP the P-PSP_DR increases but the OIS_DR decreases. In contrast, both the OIS_DR and P-PSP_DR increase when K⁺_o is elevated to 8 mM. These observations from slices from normal subjects are interpreted to indicate that the OIS_DR mainly reflects cell volume and light scattering changes associated with DH_s astrocyte uptake of K⁺ and glutamate (GLU). In slices from subjects that received an intracutaneous injection (i.c.) of formalin 3-5 days earlier both the OIS_DR and the response of the DH_s ipsilateral to the injection site to 100 msec local application (via puffer pipette) of 15 mM K⁺ or 100 µM GLU are profoundly reduced, and the normally exquisite sensitivity of the DH_s to elevated K⁺_o is decreased. Considered collectively, the observations raise the possibility that impaired regulation of DH_s K⁺_o and GLU_o may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities that develop following i.c. formalin injection.