Accurate measurement of serum glycoside concentrations is now possible. Following intravenous administration, concentrations fall rapidly (digoxin, three to four hours; digitoxin, four to six hours) through levels which, in the steady state, are seen in intoxicated patients to a slow excretion slope within the "therapeutic" concentration range (< 3 ng/ml digoxin; < 40 ng/ml digitoxin). The range of values 24 hours after an identical dose in man is narrow with digoxin (n = 6) and wide after digitoxin (n = 10). Pulse rate response and serum concentration were related in five patients with atrial fibrillation. In six intoxicated patients, electrocardiographic signs of overdosage lingered one to several days after concentrations became normal in four patients. Glycoside concentrations in the blood are related to their cardiac effects during steady state conditions. They can be misleading during unstable periods when plasma/tissue ratios may be fluctuating.