The diameters of nerve fibres in the sheep trochlear nerve were measured and they fell into two distinct groups. 2. Selective stimulation of the small diameter group of nerve fibres gave rise to a slow contraction of the superior oblique muscle which was attributed to multiply innervated muscle fibres. 3. The slow fibre contraction following a single stimulus to the small diameter nerves had a rise time of 20-50 msec and was 150-980 mg in size. On repetitive stimulation, a maximum slow fibre contraction was developed at 120-140 Hz, and maximum velocity of tension development at 170-200 Hz. 4. The maximum slow fibre tetanic tension was 7-0-12-3 g, which amounted to 5-3% of the whole muscle tetanic tension, while the maximum contracture following an injection of suxamethonium was 7% of the whole muscle tetanic tension. 5. The slow fibres were very resistant to fatigue and their contraction increased the resistance of the muscle to stretching. 6. Discrepancies from earlier work and the possible significance of the slow fibres are discussed.
