While clinical characteristics of diabetic painful neuropathy are well described, the underlying electrophysiological basis of the exaggerated painful response to stimuli, as well as the presence of spontaneous pain, are poorly understood. In order to elucidate peripheral contributions to painful diabetic neuropathy, we quantitatively evaluated the function of C-fibers in a rat model of painful diabetic neuropathy, diabetes induced by the pancreatic beta-cell toxin streptozotocin. While there was no significant effect of diabetes on conduction velocity, mechanical threshold or spontaneous activity, the number of action potentials in response to sustained threshold and suprathreshold mechanical stimuli was significantly increased in the diabetic rats. Moreover, there was a clustering of responses of C-fibers in diabetic rats; while two-thirds of C-fibers fired at the same mean frequency as C-fibers in control rats, one-third of C-fibers in diabetic rats were markedly hyper-responsive, demonstrating a threefold increase in firing frequency. The high-firing-frequency C-fibers in rats with diabetes also had faster conduction velocity than the low-firing-frequency C-fibers in rats with diabetes or in C-fibers in control rats. The hyper-responsiveness was characterized by a selective increase of the shortest interspike intervals (<100ms) in the burst component (first 10s) of the response to a sustained suprathreshold stimulus; in the plateau phase (last 50s) of the response to a 60-s suprathreshold stimulus, we found a selective increase of interspike intervals between 100 and 300ms in hyper-responsive C-fibers in rats with diabetes. The hyper-responsiveness did not correlate with mechanical threshold, presence of spontaneous activity or location of the fiber's receptive field. In summary, in an established model of painful diabetic neuropathy in the rat, a subset of C-fibers demonstrated a marked hyper-responsiveness to mechanical stimuli. The subset was also found to have a greater mean conduction velocity than the fibers not demonstrating this hyper-responsivity. The present findings suggest that study of individual neurons in vitro may allow elucidation of the ionic basis of enhanced nociception in diabetic neuropathy.