A survey of changes in discharge characteristics of single motor unit action potentials in elderly humans was conducted. The cohort consisted of 32 individuals ranging in age from 60 to 90 years. The comparison group consisted of 13 individuals from 20 to 35 years old. Active bipolar recordings of potentials during 5, 10, and 20 percent of maximum muscle voluntary contraction of each subject were obtained. Specially constructed fine wire bifilar recording electrodes were used, and the potentials were identified by interactive computer processing. Data processing produced 34354 interspike intervals from 286 potentials. The 60 to 69, 70 to 79, and 80 to 89 year age groups had 64, 49, and 53 identified potentials, respectively, corresponding to a total of 19144 interspike intervals for the three percentages of contractions. The comparisons had significantly shorter interspike intervals than did the older subjects. In each percentage of contractions, the 70 to 79 year old subjects had the longest mean interspike intervals, but the differences from the other groups were not statistically significant. The authors conclude that motor units in the 70 to 79 year old group tend to discharge more slowly and with more variation than do those of younger subjects. Larger motor units may be substituting for smaller motor units in 70 to 79 year old individuals.