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SUMMARY1. Changes in discharge rate of thirty-one fusimotor neurones to triceps surae muscles during long-lasting, fatiguing contractions of these muscles were studied in decerebrate cats. Discharges of fusimotor neurones were recorded from the nerve filaments. Muscle contractions were elicited by electrical stimulation of either the muscle nerves (twenty-one neurones) or the corresponding ventral roots (ten neurones), until the muscle tension fell to about 30% of its initial value.2. Early and late changes could be recognized in fusimotor discharge rate during long-lasting muscle contraction. The early changes obviously not related to muscle fatigue, consisted of an initial increase at the onset of muscle contraction and a subsequent decrease to or below the resting discharge level. The late change in discharge rate, supposedly related to muscle fatigue, was an increase developing gradually towards the end of muscle contraction, ranging at its peak from 2 to 15 impulses/s (mean value 5-5 impulses/s, n = 31) and outlasting the contraction for 20-320 s.3. When the contracting muscle was made ischaemic the late increase in fusimotor discharge rate started earlier and was maintained until the arterial clamp was removed. After severing the muscle nerves distal to the site of stimulation no changes, a slight sustained increase, or else a decrease in fusimotor discharge rate occurred during electrical stimulation of either muscle nerves or ventral roots. At its cessation the spontaneous firing rate was reassumed immediately. Stimulation of the distal stumps of the severed nerves elicited no changes in fusimotor discharge rate.4. It is proposed that the late increase in fusimotor discharge rate may appear due to autogenetic excitation of fusimotor neurones by discharges from group III and IV muscle afferent fibres provoked and/or enhanced by metabolic products liberated in muscle tissue during the fatiguing contraction. The fusimotor firing was estimated to remain elevated to a level twice that of the spontaneous activity on average for approximately 120 s after the muscle contraction. Its functional role in muscle fatigue is discussed.

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