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1. A brief downward, stepwise displacement applied to the outstretched finger gives rise to a train of approximately sinusoidal movements of it, lasting often more than 1 sec. The frequency of these waves is the same, in any one subject, as that of physiological tremor.

2. The oscillations are regular in form, and bear a constant phase relation to the applied displacement; they can be summated using an averaging computer (Biomac 1000) triggered by the mechanical stimulus.

3. The oscillations are altered in the same way as is physiological tremor by a number of factors. Cooling the arm before recording lowers the frequency, warming raises it, while the application of an arterial cuff decreases the amplitude and tends to elevate the frequency. These factors have effects of similar magnitude on both the oscillations and the tremor.

It thus appears highly likely that the waves produced by a mechanical input and physiological tremor waves are due to the same process, namely oscillation in an underdamped servo-system.

4. The oscillation is not due simply to the mechanical, die-away resonance of the finger, because bursts of muscle action potentials can be recorded in phase with the finger movements both in the wave train evoked by the mechanical displacement and during normal tremor.

5. It is concluded that physiological tremor in the 8-12 c/s band is due to oscillation in the stretch reflex servo-loop.

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