Four running tests were performed by five, normal, male adults on a motor driven treadmill. After determination of the max V̇o2 they ran on different days for 30 minutes at 82–89% maxV̇o2, 40 minutes at 74–79% maxV̇o2 and 60 minutes at 67–74% maxV̇o2. Gas exchange determinations and basilic vein lactate measurements were made for selected one-minute intervals over the duration of the runs. Periodic pH determinations, heart rates and rectal temperatures were monitored. In running at 82–89% maxV̇o2 the venous lactate levels were observed to continuously increase over the duration of the 30 minute performances for all subjects (peak mean 82.5mg%). Running at 74–79% maxV̇o2 resulted in an elevation. of lactate that generally plateaued around 45mg% by the 20th minute of the 40 minute performances. At 67–74% maxV̇o2 a small increase in lactate occured (means ranged from 18 to 29mg%) which was sustained over the 60 minute performances. The results indicate that blood LA is elevated over the duration of 30 to 60 minute runs in proportion to the aerobic demands in excess of 65 to 70% of maximum. This suggests that LA is continuously produced in work requiring 65 to 90% maxV̇o2 even when a reasonably steady state of O2 consumption is attained.