A compact, light, easy to use, and low cost instrument with technical solutions that make it suitable for wide clinical use is presented. It is specifically designed for rehabilitation of patients after heart failure. The instrument makes use of low power laser diodes, at 750 and 810 nm, and a remote fiber optic probe. Reflectance change at two wavelengths is used to determine variations of the oxygen content of tissues. A time-variant filter enhances signal to noise ratio and rejects stray light. This specific electronic device allows the use of a low-cost, small and reliable photodiode in place of a photomultiplier tube. Time division techniques is used to process both 750 and 810 signals with a single collecting fiber, photodiode, and preamplifier. The instrument output is two analog signals proportional to the reflectance intensities at 750 and 810 nm, so it is possible to acquire these signals by a PC with a standard A/D board to drive directly a chart recorder. Some clinical tests during the exercise are presented.