Normative data that summarize the energy requirements and gait characteristics of level outdoor walking were determined in 111 normal subjects between the ages of 20 and 80 years. Subjects were divided into two age groups: young adults (20-59 years) and senior subjects (60-80 years). The mean rate of oxygen consumption for young adults and senior subjects did not significantly differ, averaging 11.9 ml/kg-min for both groups. The data on heart rate paralleled the findings on oxygen consumption, averaging 100 and 103 beats/min, respectively. The net oxygen cost per meter walked for senior subjects, 0.16 ml/kg-m, was significantly greater (p less than 0.0005) than the value for young adults, 0.15 ml/kg-m, due to a decline in the average walking speed. The average gait velocity for senior subjects, 73 m/min, was statistically significantly less (p less than 0.0005) than the values for the younger adults, 80 m/min.