The energy cost and intensity of exercise performed at 0% grade were determined for walking at 2, 3, and 4 mph, running at 5, 6, and 7 mph, and walking at 2, 3, and 4 mph with ankle and/or hand weights. Subjects were young moderately trained males (4) and females (3). The energy cost per kilogram of body weight was similar between sexes, and data were combined for among-treatment comparisons. Intensity of effort and energy cost per minute and per mile were increased when weight was added during walking and were increased more with hand weights compared with ankle weights regardless of speed. The average increase in O2 uptake (ml X kg-1 X min-1 X 100 g-1 of added wt) was 0.8% for ankle, 1.3% for hand, and 0.9% for ankle and hand weights. Gross energy cost per mile during weighted walking (120-158 kcal/mile) was comparable to and in some cases exceeded that of running which was independent of speed (120-130 kcal/mile). During nonweighted walking, the energy cost (kcal/mile) was significantly greater at 4 mph compared with 2 and 3 mph which did not differ. The intensity of walking at 4 mph with ankle and hand weights was comparable to running at 5 mph.