Microneurographic studies were made of normal human muscle spindle afferent activity to evaluate the role played by these endings, and by the segmental stretch reflex, in various types of physiological tremor. Primary endings are sufficiently sensitive to respond to the minute tremulous movements normally seen in contracting muscles. Human muscle spindle endings appear to possess the same non-linear sensitivity to small stretches as has been reported for cat muscle spindles. The findings also support the notion that the segmental stretch reflex plays an important role in enhanced physiological tremor, in so far as it tends to synchronize the motor outflow in such a way as to produce rhythmical contractions properly timed to reinforce the mechanical resonant properties of muscle spindle endings, the stretch reflex also serves to damp the larger oscillations of finger or hand which result from a brisk tap or muscle twitch. There is no evidence for alpha-gamma co-activation in these enhanced physiological tremors.