1. For the diagnosis of electrolyte disorders, data on skeletal muscle composition are often valuable, but rarely available. We have therefore developed a simple and rapid needle biopsy procedure for the determination of the concentrations of K + , Na + , Mg 2+ and Na + , K + -pumps in muscle. 2. Using a Bergström needle, biopsies weighing around 25 mg were taken from the vastus lateralis muscle of 18 normal subjects (aged 44–86 years) and extracted with trichloroacetic acid (TCA). The concentrations of K + , Na + and Mg 2+ were 90.7 ± 1.8, 31.9 ± 1.6 and 9.5 ± 0.2 μmol/g wet wt., respectively (means ± se ). 3. The TCA extraction gave the same values as digestion with 65% HNO 3 or 35% H 2 O 2 , could be used over the weight range 10–55 mg and showed a Mg 2+ recovery of 101.7%. 4. The concentration of Na + , K + -pumps was quantified as the total capacity for [ 3 H]ouabain binding. In vastus lateralis biopsies obtained from six normal subjects the mean value was 258 ± 16 pmol/g wet wt. 5. Comparison of the concentrations of K + , Mg 2+ and [ 3 H]ouabain-binding sites in samples obtained post mortem showed modest variation among different muscles with varying fibre composition. 6. Measurements of the concentrations of K + , Na + , Mg 2+ and Na + , K + -pumps in duplicate biopsies of the vastus lateralis yield values which seem representative for the total pool of skeletal muscle fibres and can be performed within a few hours. 7. Measurements on patients receiving chronic diuretic treatment demonstrated that these procedures allow the detection of reduced levels of K + , Mg 2+ and Na + , K + -pumps in skeletal muscle, as well as a partial recovery after oral Mg 2+ supplementation.