Abstract.Eighteen patients with atrial fibrillation were given digoxin 0.13 mg twice daily for 3 weeks and β‐methyldigoxin 0.10 mg twice daily for another 3 weeks. At the end of each 3 week period an exercise test was performed and the effects on the heart rate of the two drugs were compared. No difference in heart rate was obtained at rest, whereas the heart rate after 6 min of exercise was higher during treatment with digoxin (131 beats/min) than when the patients were taking β‐methyldigoxin (124 beats/min). There were no significant differences between digoxin and β‐methyldigoxin in their effects on the ECG (R‐R intervals, T‐wave, Q‐T duration). The plasma concentrations of the two glycosides were determined by radioimmunoassay and by86Rb‐uptake inhibition assay. Comparable plasma concentration values (1.0 ng/ml for digoxin, 1.1 ng/ml for β‐methyldigoxin, mean values) were obtained by radioimmunoassay, but the86Rb‐technique gave significantly higher values (mean 1.5 ng/ml) for β‐methyldigoxin.It is concluded that β‐methyldigoxin is equal to digoxin for producing slowing of the heart rate in patien