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Department of Physiology, University of Bristol Medical School.

1. It was confirmed that in the fourth deep lumbrical muscle of the rat the number of muscle fibres at birth is about half that in the adult. 2. The average number of slow-myosin-containing (S) fibres (as determined by specific antibody binding) remains constant from birth to adulthood. Therefore it is likely that all the muscle fibres generated post-natally are type F (i.e. slow-myosin-free). 3. A comparison in the electron microscope between transverse mid-belly sections from new-born (day 0) and 4 day muscles showed many muscle fibres or myotubes to be intimately associated with other fibres and cells of other types in the new-born, but to be much less closely grouped at 4 days. 4. A full cell count was obtained from electron microscopy of a mid-belly section of a lumbrical muscle at birth. 5. Cross-sectional area measurements in the light microscope at 3-5 days and in the adult showed that at 3-5 days on average the S fibres have a greater cross-sectional area than the F fibres. This is reversed in the adult where the S fibres are the smaller. At 3-5 days the range of cross-sectional areas of F fibres is much wider than for S fibres. Some F fibres are among the largest fibres in the muscle. 6. It is argued from the data that the motor units of adult muscle, which are homogeneous with respect to muscle-fibre types, are produced by selective withdrawal of neonatal motor-unit contacts during developmental synapse elimination.