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1. Isosmetric contractions of cat flexor digitorum longus whole muscles and of functionally isolated motor units have been measured under conditions similar to those used by Buller & Lewis (1965a).

2. Motor unit twitch time to peak was inversely related to axonal conduction velocity. The logarithm of tetanic tension was directly related to conduction velocity. These relationships suggest that each motoneurone has an influence on the muscle fibres which it innervates.

3. The ratio of twitch to tetanic tension was directly related to the time to peak of the motor unit. This fact might be explained by variation between motor units of the duration of `active state'.

4. The muscle length at which tension was maximal varied between motor units and the optima were found over the range of muscle lengths which could occur in the body. Slow motor units had longer optimal lengths.

5. The sample of motor units was considered to be unbiased because the distribution of axon conduction velocities was compatible with reported motor fibre diameter spectra of the muscle nerve. The mean motor unit tetanic tension gave a reasonable estimate of the number of -motor axons in the muscle nerve. Twitch tensions gave a value that was 40% higher.

6. Motor unit and whole muscle data were in good agreement for length-tetanus tension curves, for times to peak and for twitch-tetanus ratios at long muscle lengths.