Myelin in femoral nerve segments obtained at autopsy was isolated quantitatively by a series of discontinuous and continuous flotation procedures. The total amount of myelin isolated from these nerves was expressed as the sum of cholesterol, glycolipid, phospholipid, and protein and averaged 2.6+/-0.4 mg/g in a group aged 60-77 yr compared with 10.8+/-1.9 mg/g of nerve in a group aged 35-58 yr. The lower value in the older group remained apparent whether the myelin content was related to the whole nerve segment, its unit length or weight. This indicates that the decrease is an absolute one, not related to a change with aging in the nonmyelin content of nerve. No qualitative differences in myelin lipids were found between the two groups. Protein content was, however, significantly higher in the older group (34 and 28.7% of the total myelin weight, respectively). The decrease in myelin content with aging, observed by direct measurement in this study, may be the structural counterpart to age related alterations in peripheral nerve function-decreased conduction velocity, and impaired appreciation of vibration.