To determine the change in muscle oxygenation in response to progressively increasing work rate exercise, muscle oxyhemoglobin + oxymyoglobin saturation was measured transcutaneously with near infrared spectroscopy in the vastus lateralis muscle during cycle ergometry. Studies were done in 11 subjects while gas exchange was measured breath-by-breath. As work rate was increased, tissue oxygenation initially either remained constant near resting levels or, more usually, decreased. Near the work rate and metabolic rate where significant lactic acidosis was detected by excess CO2 production (lactic acidosis threshold, LAT), muscle oxygenation decreased more steeply. As maximum oxygen uptake (VO2max) was approached, the rate of desaturation slowed. In 8 of the 11 subjects, tissue O2 saturation reached a minimum which was sustained for 1-3 min before VO2max was reached. The LAT correlated with both the VO2 (r = 0.95, P < 0.0001) and the work rate (r = 0.94, P < 0.0001) at which the rate of tissue O2 desaturation accelerated. These results describe a consistent pattern in the rate of decrease in muscle oxygenation, slowly decreasing over the lower work rate range, decreasing more rapidly in the work rate range of the LAT and then slowing at about 80% of VO2max, approaching or reaching a minimum saturation at VO2max.