Pain and hyperalgesia can occur when the dorsal root ganglion (DRG) and its roots are deformed mechanically in association with injuries or diseases of the spine. To evaluate the electrophysiological changes that contribute to this sensory pathology, intracellular recordings were obtained in vitro from DRGs that had received a chronic mechanical compression [chronic compression of DRG (CCD)]. The compression was produced by inserting L-shaped rods ipsilaterally into the intervertebral foramina, one at L(4) and the other at L(5) in rats 1-14 days before the recording. Control rats received a sham operation. Postoperatively, the threshold force applied by punctate stimulation of the plantar surface of the hind paw decreased significantly on the foot ipsilateral to the CCD (mechanical hyperalgesia) but changed little on the contralateral foot or on either foot for control rats. DRG somata were viewed through a microscope during recording and classified as small, medium, and large according to their diameters. CCD cells in each size category were more excitable than those of comparable size from control rats as judged by the significantly lowered threshold currents and action potential voltage thresholds. Spontaneous activity was recorded in 11% of all the CCD cells tested. The spontaneous activity and/or changes in both the threshold currents and action potential thresholds were observed as early as one day after injury. The association of cutaneous hyperalgesia with changes in the electrophysiological properties of DRG cells suggests a possible role for intrinsic alterations in the membrane properties of compressed DRG cells in the production and persistence of chronic pain after certain spinal injuries or pathologies of the spine.