Blood samples and skeletal muscle biopsies (m. quadriceps femoris, vastus lateralis) were taken from seven healthy subjects for analysis of serum and skeletal muscle digoxin concentrations by radioimmunoassay using a percutaneous needle biopsy technique for muscle sampling. The subjects were investigated on two digoxin dose levels and on the third day after withdrawal of digoxin. It was found that the skeletal muscle/serum digoxin ratio was significantly higher than the corresponding ratio obtained in a previous study with muscle sampling (m. rectus abdominis) from patients during open heart surgery. The present study indicates a significant correlation between the digoxin concentrations in serum and skeletal muscle as well as between cardiac effect, measured by changes in QS2I, and skeletal muscle digoxin concentration. A doubling of the digoxin dose gave a proportional increase in skeletal muscle digoxin concentration. The magnitude of the estimated half-life of skeletal muscle digoxin was the same as previously reported for serum or plasma digoxin.