A new method of biotelemetry is proposed, which realizes non-restrained acquisition of biological signals using infrared light diffused out from animal skin. This technique is fundamentally different from conventional telemetry techniques which use radiofrequency electromagnetic waves. An optical transmitter, which consists of some LEDs is implanted beneath the skin. Biological information such as an ECG signal is collected in vivo and the light emitted from the LEDs is modulated by the signal. The light transmitted through the skin is strongly diffused and spread out in a wide angle. The light is collected by remotely located optical receivers and biological information is obtained by demodulating the signal. Thus, the biological information in vivo is obtained without restraining the object with cables or optical fibers. In order to verify the possibility of the above techniques, a system which obtains an ECG from a laboratory animal has been developed. An infrared light was chosen because it is invisible, and has high transmittance through the skin. In addition, there are wide variety of light sources and detectors available in the range of near-infrared. Considering the stability of the communication link and the power consumption of the implanted transmitter, a PFM (pulse frequency modulation) technique was used. Using the system the transcutaneous ECG telemetry was successfully performed. In the experiment the indirect light, reflected and scattered by walls, a ceiling and a floor, was shown to be useful for optical telemetry. Light transmission patterns through the skin of a mouse were measured. The results suggest the possibility of telemetry even from the deep part of the body cavity. It is concluded that recent progress in optical and electronic technology has reached the point where transcutaneous optical telemetry of biological signals has become possible.