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1. Isometric contractions of the medial gastrocnemius (fast switch) and soleus (slow twitch) muscles were recorded in kittens ranging in age from 3 to 112 days, as well as in adult cats. 2. It was confirmed that the speed of contraction of the gastrocnemius muscle becomes progressively faster during the first few weeks after birth, whereas contraction times of the soleus muscle show little changes or a slight prolongation during the period of post-natal development. 3. The properties of gastrocnemius (fast alpha) and soleus (slow alpha) motoneurons were examined with intracellular electrodes in kittens at three different stages; 16-20, 61-71 and 100-112 days in age. 4. The axonal conduction velocities of both gastrocnemius and soleus motoneurones increased monotonically throughout the period of development and showed no correlation with post-natal changes in contraction times of the innervated muscles. 5. The duration of after-hyperpolarization in soleus motoneurones became progressively longer with age, while that in gastrocnemius motoneurones remained virtually unchanged during development. 6. The relation between the duration of after-hyperpolarization and the axonal conduction velocity in kitten motoneurones was similar to that observed in axotomized motoneurones of adult cats. 7. It is suggested that fast and slow alpha motoneurones show postnatal differentiation in terms of the duration of after-hyperpolarization and that axotomy leads to 'dedifferentiation' of the motoneurone properties. 8. Post-natal changes in the contractile properties of skeletal muscles were independent of the changes in the duration of after-hyperpolarization of the innervating motoneurones. However, it remains uncertain whether muscle differentiation is independent of the discharge pattern of the innervating motoneurones.

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