SummaryAssay of digitalis glycosides in man can now be done easily. In what circumstances is such a measurement meaningful?First of all, assay methods have allowed studies that have furthered our knowledge about digitalis and its use. These methods have drawn attention to the problem of bioavailability of digoxin : differences between different oral preparations, but also between different tablet brands are certainly important for the practical use of the drug. We know also more about the use of digitalis glycosides, mainly digoxin, in renal failure and the data derived from studies of plasma levels have allowed us to set up equations for calculating the daily digoxin requirement as a function of the degree of renal failure.Another aspect is whether it makes sence to measure the plasma levels of a glycoside to improve therapy in the individual patient. If the patient is doing well, measuring a plasma level does not learn much, except if the patient is in renal faillure. A plasma level can be helpful if a patient does not respond to therapy, or if toxicity suspected.Plasma levels of digitalis glycosides can be helpful, but only in some circumstances.