Abstract 1. 1. The phosphatidyl compounds containing inositol, serine, ethanolamine and choline, the phosphatidal (plasmalogen) compounds containing serine, ethanolamine and choline, a lipid containing “polyglycerophosphate”, the protein-bound phosphoinositides and an alkali-acid stable fraction have been isolated and separated in both normal and denervated rat-gastrocnemius muscles and their labelling patterns from 32P i determined. 2. 2. The denervated muscle incurred losses on a whole muscle basis in all the phospholipids studied, with the exception of phosphatidal choline. The losses, however, ranged from very marked for protein-bound phosphoinositides and “polyglycerophosphate” to slight for phosphatidal ethanolamine and phosphatidal serine. 3. 3. The normal rat gastrocnemius actively incorporated 32P i into each of the phospholipids. For all fractions except phosphatidyl inositol and the protein-bound phosphoinositides, there is a progressive rise in incorporation with time. Phosphatidyl inositol showed rapid initial labelling to a plateau corresponding to approx. 20% of the labelling of muscle-inorganic phosphate. The protein-bound phosphoinositides also showed rapid initial labelling from muscle-inorganic phosphate which eventually reached a plateau at 12 h. The patterns of labelling of both of these fractions is consistent with the presence of more than one pool with at least one pool being labelled rapidly from inorganic phosphate. 4. 4. Fifteen days after denervation, a number of changes are observed in the incorporation of 32P into the individual phospholipids with time. By 96 h there was an increase in the specific activity of each of the phospholipids relative to the specific activity of muscle-inorganic phosphate. At 12 h these increases were not evident. 5. 5. After denervation the incorporation of label expressed as a corrected relative uptake ( i.e. a measure of 32P i incorporated into whole denervated muscle relative to 32P i incorporated into whole normal muscle corrected for differences in the specific activities of inorganic phosphate in normal and denervated muscle) was decreased in all phospholipids with the exception of the phosphatidal compounds. 6. 6. Comparison of the ratio of specific activities of each phospholipid fraction with that for muscle-inorganic phosphate showed that incorporation of 32P i was increased into phosphatidal ethanolamine and the “polyglycerophosphate” containing lipid and was decreased into phosphatidal choline and protein-bound phosphoinositide. When restricted to a unit weight of muscle these results have been interpreted in terms of changes either in the enzyme activity concerned with incorporation of 32P i or in the size of the product pool.