The effect of intramuscular Digoxin on 20 cases is analysed. The glycoside is rapidly absorbed from muscle, and the results indicate that the intramuscular route is an efficient substitute for intravenous therapy when urgent parenteral digitalisation is required. In undigitalised adults a suitable initial dose is 1 mg. and maximum therapeutic effect is likely within six hours. In auricular fibrillation initial ventricular slowing may be mainly vagal. Concurrent mercurial diuresis may enhance the therapeutic effect of intramuscular Digoxin probably due to increased absorption. Intramuscular Digoxin may be painful and this may limit its value for maintenance therapy. The pain appears to be due to a direct action of Digoxin on voluntary muscle.