Impulses of human single muscle spindle afferents were recorded from the m. extensor carpi radialis, while 1 Hz sinusoidal movements for a wide range of amplitudes (0.05-10 deg, half of the peak-to-peak amplitude) were imposed at the wrist joint. The response was considered as linear when the discharge was approximately sinusoidally modulated. The linearity was further checked by a linear increase in the response with the amplitude and a constancy of the phase and mean level. Fifteen of 25 primary afferents were active at rest with a mean rate of 10.6 impulses s-1 (median). The linear response to sinusoidal stretching was limited to amplitudes lower than about 1.0 deg. The sensitivity was 5.6 impulses s-1 deg-1 (median) in the linear range and decreased at larger amplitudes. The other 10 primary afferents were silent at rest and lacked a linear response at low amplitudes. Nine secondary afferents were active at rest with a mean rate of 9.5 impulses s-1. The linear range extended up to about 4.0 deg with a sensitivity of 1.4 impulses s-1 deg-1. In the linear range, the phase advance of the response to sinusoidal stretching was about 50 deg and was similar between the two types of spindle afferents. In primary afferents, the phase advance increased to nearly 90 deg outside the linear range. The findings suggest that high sensitivity to small stretches is important in determining primary afferent firing during natural movements in intact humans.