The myelin fluidity in peripheral nerves from normal subjects and patients with neuropathy has been studied. Nerve biopsies were made in subjects without neuropathy and in patients with Guillain-Barré, alcoholic or diabetic polyneuropathies and heredodegenerative diseases. The fluidity was studied using a stearic acid spin label with a nitroxide radical at C12 (12-doxylstearic acid) which was introduced by exchange between spin label-charged BSA and nerve myelin. Spectra was obtained with a VARIAN E 109 ESR spectrometer, at temperatures from 5 to 45 degrees C. The ultrastructure of biopsied nerves was also studied. The electron spin resonance (ESR) spectra were all typical of a partially immobilized nitroxide with a distance between outer hyperfine lines (2T//) of about 50 G, at room temperature. The curve 2T// against temperature shows two critical temperatures: 10 and 30 degrees C for all nerves. No correlation could be established between ultrastructure and fluidity of nerves. However, the myelin of nerves from diabetic patients showed a significant increase in viscosity. This physicochemical modification may represent an initial alteration of the myelin sheath, and demands further investigation. No modification of the myelin fluidity was observed in any of the other nerves studied.