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1. These experiments were based on the recent observation of Gladden (1976), that acetylcholine (ACh) when applied to the isolated cat muscle spindle caused contraction of the dynamic and the static nuclear bag fibres, and not of the nuclear chain fibres, and that the dynamic nuclear bag fibre had the lower threshold to ACh than the static nuclear bag fibre. Subsequently, suxamethonium (SCh) has been shown to have similar effects on the intrafusal muscle fibres (Gladden & McWilliam, 1977). 2. In these experiments, the response of cat soleus muscle spindle primary, secondary and 'intermediate' sensory endings to repetitive ramp stretches during continuous slow infusions of SCh were studied. The changes observed are interpreted on the basis of the known action of SCh on the intrafusal muscle fibres of the isolated spindle. 3. Primary sensory endings, with afferent axon conduction velocities above 80 m/sec, were activated during SCh infusion in three stages. In Phase I of excitation, a gradual facilitation of the discharge of the Ia endings was seen, without potentiation of the dynamic or length sensitivity to stretch. In Phase II of excitation, the dynamic sensitivity of the Ia endings increased very markedly. In Phase III of excitation, an increase in length sensitivity was superimposed on the already enhanced dynamic sensitivity. 4. Secondary sensory endings, with afferent axon conduction velocities below 60 m/sec, only experienced a gradual facilitation of their discharge during SCh infusion, similar to the Phase I effects of SCh on primary endings. 5. The majority of 'intermediate' sensory endings, with afferent axon conduction velocities between 60 and 80 m/sec, were activated by SCh either in the same way as primary endings, or in the same way as secondary endings. However, a significant number of these sensory endings behaved in a truly intermediate manner during SCh infusion (cf. Rack & Westbury, 1966), and may represent an intermediate form of spindle sensory ending. 6. The afferent axon conduction velocities of these truly intermediate sensory endings were restricted to the range 69-77 m/sec. All the 'intermediate' endings which were activated in a primary-like manner had afferent axons conducting at velocities greater than 74 m/sec. 'Intermediate' endings which were activated in a secondary-like manner had afferent axon conduction velocities below 72 m/sec. 7. The probable contribution of the static and dynamic nuclear bag fibres to the discharge of each type of spindle sensory ending is discussed.
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