The spin labels, 5-nitroxide stearic acid and 16-nitroxide stearic acid were incorporated into whole sciatic nerves dissected from normal, quaking, jimpy and trembler mice. With 5-nitroxide stearic acid, we have studied the thermal variation of the maximal apparent coupling constant (T) between 0 degrees C and 50 degrees C. Within this range of temperatures, we obtained identical values of 2 T for nerves from normal and jimpy mice, whereas 2 T was smaller for nerves from quaking and trembler mice. With 16-nitroxide stearic acid, composite spectra were recorded, particularly in the high-field range. A line characteristic of myelin was clearly observed in the spectra of nerves from normal and jimpy mice; its intensity was somewhat less in nerves from quaking mice and much less in spectra from trembler mice. A shoulder in the principal highfield line of the spectrum is modified only with nerves from jimpy mice. The results agree well with those obtained by electron microscopy, which reveal normal myelination in nerves from jimpy mice, a slight modification of the myelin from those of quaking mice and a practically complete demyelination in peripheral nerves from trembler mice. However, the structure of the nerves of jimpy mice also seems to be modified at an, as yet, undetermined level.