Little information is available on the pH sensitivity of the excitability properties of mammalian axons. Computer-assisted threshold tracking in humans has helped to define clinically relevant changes of nerve excitability in response to hyperventilation and ischaemia, but in vivo studies cannot directly differentiate between the impact of pH and other secondary factors. In this investigation, we applied an excitability testing protocol to a rat saphenous skin nerve in vitro preparation. Changes in extracellular pH were induced by altering pCO(2) in the perfusate, and excitability properties of large myelinated fibres were measured in the pH range from 6.9 to 8.1. The main effect of protons on nerve excitability was a near linear increase in threshold which was accompanied by a decrease in strength-duration time constant reflecting mainly a decrease in persistent sodium current. In the recovery cycle, late subexcitability following 7 conditioning stimuli was substantially reduced at acid pH, indicating a block of slow but not of fast potassium channels. Changes in threshold electrotonus were complex, reflecting the combined effects of pH on multiple channel types. These results provide the first systematic data on pH sensitivity of mammalian nerve excitability properties, and may help in the interpretation of abnormal clinical excitability measurements.