The effect of denervation on the lipid metabolism and on the activity of (Na + -K + )ATPase isoforms from the membrane fraction P 3 , which corresponds to the innervated electrocyte membrane, was evaluated. On a discontinuous sucrose gradient, normal P 3 membranes exhibit a bimodal (“a” and “b ” bands) distribution of the (Na + -K + )ATPase activity, which upon denervation changes to an unimodal (“c” band) distribution. Using these fractions, which have a higher (Na + -K + )ATPase activity, we characterized the lipids at the hydrophobic protein surface boundary, (i.e., the bulk lipids that surround the protein). The results confirm that these lipids consist of phospholipids and cholesterol. The quantitative composition of the phospholipids is similar for both isoform fractions obtained from the discontinuous gradient of normal membranes, with phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine representing about 90% of the total phospholipids. Sphingomyelin, phosphatidylinositol, diphosphatidylglycerol and phosphatidic acid were in the minority. However, in the single band obtained after denervation, the three major phospholipid components decreased to 70% of the total, and a significant increase in the other phospholipids and in cholesterol was observed. The high cholesterol content of the denervated fraction may confer membrane stabilization, as it is likely to cause a decrease in the membrane fluidity and consequently in the enzyme activity.