We have developed a 200-channel imaging system that enables measurement of changes in oxygenation and blood volume and that covers a wider area (45 cm 15 cm) than that covered by conventional systems. This system consisted of 40 probes of five channels, a light-emitting diode (LED) driver, multiplexers and a personal computer. Each probe was cross-shaped and consisted of an LED, five photo diodes, and a current-to-voltage (I-V) converter. Lighting of the LEDs and acquisition of 200-channel data were time-multiplexed. The minimum data acquisition time for 200 channels, including the time required for calculation of oxygenation and monitoring of a few traces of oxygenation on a computer display, was about 0.2 s. We carried out exercise tests and measured the changes in oxy- and deoxy-hemoglobin concentrations in the thigh. Working muscles in exercises could be clearly imaged, and spatio-temporal changes in muscle oxygenation during exercise and recovery were also shown. These results demonstrated that the 200-channel imaging system enables observation of the distribution of muscle metabolism and localization of muscle function.