Maximum peak-to-peak fibrillation potential amplitude was measured in 69 subjects between 7 days and 10 1/2 years post complete or partial peripheral nerve injury. Mean amplitude during the first 2 months was 612 muV; third and fourth months 512 muV, fifth and sixth months 320 muV. After the first year, no population of fibrillation potentials greater than 100 muV was recorded. The sciatic nerve was sectioned in 13 guinea pigs and animals studied up to 17 weeks. Fibrillation potential amplitude in gastrocnemius muscles declined paralleling that in humans. By the end of the study, type I fibers had lost almost half of their initial diameter and type II fibers had atrophied more than twice this amount. Fibrillation potential amplitude may be useful in estimating the time post nerve injury and appears to correlate with the surface area and fiber diameter of a type I muscle fiber.