Twenty young men marched 6 and 12 km with a well-fitted back-pack load of 30 or 35 kg. Each subject served as his own control. No significant increase in mean heart rate, rectal temperature, or decrease in mean VO2 max and serum levels of glucose and muscle enzymes were recorded in the groups marching 6 km with 30 and 35 kg. Significant differences in the increases in mean heart rate, the decreases in VO2 max and the changes in blood glucose were noted between the two groups carrying 30 and 35 kg for 12 km. These significant differences were also supported by the subjective feelings of the volunteers. The present study shows the optimal back-pack load for healthy young men, marching at 6 km/hr on a paved level road to be 30 kg for 12 km and 35 kg for 6 km without considering the task too difficult and with no significant decrease in VO2 max. The results are relevant to hiking, rescue assignments, and military missions.