Multiple nerve excitability measurements were used to investigate axonal membrane properties in patients with chronic renal failure (CRF). Nine patients were studied during routine haemodialysis therapy. The median nerve was stimulated at the wrist and compound muscle action potentials recorded from abductor pollicis brevis. Stimulus-response behaviour, strength-duration time constant, threshold electrotonus, current-threshold relationship and recovery cycle (refractoriness, superexcitability and late subexcitability) were recorded using a recently described protocol. In six patients, sequential studies were performed before, during and after haemodialysis. All patients underwent standard electrolyte and renal function tests before and after haemodialysis. Before dialysis, there were significant abnormalities in axonal excitability: reduced superexcitability; increased accommodation to depolarizing and hyperpolarizing currents; and a steeper current-threshold relationship compared with normal controls. These excitability parameters are the most sensitive to membrane potential and the abnormalities, which were all reduced by haemodialysis, closely resembled those in normal axons depolarized by ischaemia. Before dialysis, the excitability parameters correlated significantly with serum potassium (range 4.3-6.1 mM), but not with other markers of renal dysfunction: patients with normal axonal resting potentials had normal serum potassium, although urea and creatinine were elevated. We conclude that nerves are depolarized in many CRF patients and that the depolarization is primarily due to hyperkalaemia.