An assay of serum digoxin and other cardiac glycosides by displacement of3H-ouabain from Na-K ATPase is described. It is rapidly set up by any laboratory with a liquid scintillation counter. It can also assay digitalis leaf, digitoxin, and gitalin (Gitaligin), and has uncovered unsuspected administration of them.The assay takes less than 1 hour. No quench correction of counts is required. One technician can assay 30 samples/day with results that afternoon. No interfering compounds have been found in over 1,800 assays. It is unaffected by lung scans.Incidence of toxicity from digoxin is 10%, 25%, and 50% at serum levels of 1.4, 2.0, and 2.9 ng/ml, respectively. Toxic levels of digitalis and digitoxin are usually over 40 ng/ml.Serum digoxin levels are also highly correlated with computed total-body concentrations of digoxin, obtained from data of dosage, route of administration, and the patient's weight and renal function.Physicians may now select a serum digoxin level and computed total-body concentration of digoxin which are associated with an incidence of arrhythmias which they feel is reasonable for the urgency of each patient's clinical situation. These selected serum levels are associated with specific loading and maintenance dosage regimens, individualized to that patient's body weight, renal function, and route of digoxin administration.