The effect of Corwin, a new oral beta, partial agonist, on the ventricular response to atrial fibrillation was studied in digitalised patients during 24 hour ambulatory electrocardiography and during exercise on a treadmill in a double blind placebo controlled crossover trial. Corwin reduced the maximum heart rate during exercise from 162(16) beats/min to 120(9) beats/min and reduced the peak heart rate during ambulatory electrocardiography from 113(11) to 90(6) beats/min consistent with a beta adrenoreceptor antagonist action at higher levels of sympathetic nervous system activity. Minimum heart rate during ambulatory electrocardiography was increased from 62(5) to 70(5) beats/min indicating that at lower levels of sympathetic activity the drug acts as a beta agonist. The drug increased exercise tolerance significantly. Serum digoxin concentrations were not affected by the drug. Thus Corwin appears to be effective in stabilising heart rate during atrial fibrillation both at rest and during exercise in digitalised patients.