After extraction of myocardial and skeletal muscle biopsy and autopsy specimens tissue glycoside concentrations can be determined by radioimmunoassay. Total tissue extraction of digoxin and beta-methyl-digoxin varies between 87 and 95%, the variation coefficient for repeated determinations is 10.2%. Glycoside concentrations of left ventricular papillary muscle obtained after mitral valve replacement were 69.0 +/- 25.05 ng/g with a tissue to serum relation of 46.6 +/- 8.96:1 and a correlation coefficient of r = 0.8442. In autopsy left ventricular papillary muscle glycoside concentrations were 105.2 +/- 27.35 ng/g with an almost identical tissue to serum relation of 46.2 +/- 9.57:1 and a corresponding serum concentration of 2.3 +/- 0.63 ng/ml. In adults glycoside concentrations of autopsy specimens of the right ventricle were significantly lower by 28 to 30% than those of the left ventricle. Glycoside concentrations of skeletal muscle specimens (m. pectorialis major) were 14.7 +/- 10.35 ng/g with a tissue to serum relation of 9.7 +/- 3.00:1 (r = 0.8377), which corresponds to approximately 1/5 to 1/4 of the concentrations of the left ventricular myocardium.