Warfarin is the first synthetic anticoagulant which is clinically available for administration by other than the oral route. Administered intramuscularly in a single dose of 75 mg., the peak effect of hypoprothrombinemia was observed on the second and third day and the total therapeutic effect lasted as long as five days. Injections were made in the upper outer quadrant of the buttocks or, in a few cases, in the deltoid region. The drug was well tolerated by the patients, and serial blood pressure readings showed no significant change. Warfarin was administered intramuscularly to patients in their homes. This presented no unusual problem requiring hospital facilities.