Abstract : The objective with this study is to develop and calibrate an invasive data logger to measure muscle O2 saturation in large, freely diving whales. We intend to use this data logger to measure muscle O2 saturation and determine how blood flow to muscle is altered during diving. These data will be important to determine if muscle blood flow is reduced during diving, and important to estimate how the dive response affects muscle N2 levels and the risk of decompression sickness (DCS).