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1. Indirect evidence on the time course of intrafusal events associated with stimulation of a static fusimotor axon was obtained by analysis of the responses from muscle spindle secondary endings of the cat.

2. In many instances, a single impulse in a single static fusimotor axon to a muscle spindle evoked a transient increase in discharge of the secondary ending the duration of which ranged from 30 to 60 msec with a rising phase of 6-20 msec. Increasing the length of the muscle increased the slope of the rising phase and augmented the magnitude of the response.

3. Different static fusimotor axons produced different effects upon secondary endings. A single impulse in some fusimotor axons did not produce an effect while repetitive stimulation produced a smooth increase in discharge frequency. When efficacious, a single fusimotor impulse evoked approximately a similar duration of increased discharge from a secondary ending and a primary ending of the same muscle spindle. The relative magnitude of effects of a given fusimotor axon on primary and secondary endings of a spindle varied.

4. During partial curarization the changes in the discharges of a secondary ending evoked by stimulation of a single static fusimotor axon occurred in a step-like fashion.

5. These results are interpreted as supporting the idea that static fusimotor axons induce twitch-like contractions of intrafusal elements (nuclear chain) through the intermediary of propagated action potentials.