The impulse response and the receptor potential 01 the isolated muscle spindle of the frog were recorded during sinusoidal stretching at frequencies from 0.1 c/s to 100 c/s. Attention was focused on the changes of the responses in the initial phase of stimulation before the steady state was reached. In the course of sine wave stimulation the impulse response of the spindle undergoes regular changes characterized by an increase in threshold and latency of the individual impulses, a decrease in phase lead and peak frequency and a reduction in number of spikes in each cycle. The changes are most pronounced during the first 2–5 cycles, and a steady state with a fixed impulse patterning is usually reached after 10–15 cycles. The receptor potential evoked by a sine wave stretch is distorted in comparison with the waveform of the stimulus in having a relatively faster rise and slower decay. With changes in frequency and amplitude of stimulation it undergoes characteristic alterations in phase lead and configuration. When superimposed upon a constant stretch or a linearly rising stretch, sinusoidal stimulation produces responses which increase in amplitude with the underlying stretch. These observations provide evidence that the sensory endings responsible for the transducer action do not obey linear transfer functions.