1. Measurements have been made of the size and number of muscle fibres and number of nuclei in a small indicator muscle (m. flexor digiti V brevis) in the fore-foot of the pig. Wellnourished, 10-d-old and 1-year-old animals were studied, as well as 1- and 2-year-old animals that were severely energy-deficient and protein-deficient.
2. The normal 1-year-old animals had much larger muscle fibres, with more nuclei, than the pigs in any of the other groups.
3. There were no significant differences between the numbers of fibres in the muscles of pigs in any of the four groups.
4. There was a significant difference between the number of fibres in the muscles of pigs coming from different litters, irrespective of their dietary history after birth. This suggests that the number of fibres is determined genetically before birth, and all that can take place after birth is an alteration in size.