An in-vivo method for the assessment of the latency of muscular contraction is presented. It is based on acoustic registration of muscular thickening by a microphone. It was found that the recorded sound wave is often preceded by a small wave of opposite polarity (flattening of the muscle belly) which is an acoustic correlate of the latency relaxation. Strong direct stimulation produced another small sound wave at the beginning of the latent period. The physiological significance of these findings is discussed.