One-stage prothrombin times of normal and of factor VII-deficient beagle plasma were determined with two types of beagle brain thromboplastin, one prepared from normal beagles and the other from factor VII-deficient beagles. There was little difference between the reagents in the prothrombin times obtained for normal plasma. However, when factor VII-deficient plasma was tested, reagent prepared from factor VII-deficient beagles gave considerably longer prothrombin times than were obtained with the normal reagent and the difference increased with increasing reagent concentration to a maximum at 140 mg/ml. Prothrombin times of a series of mixtures of normal and factor VII-deficient plasma indicated that the presence of only 1/90 part of normal plasma was necessary to compensate for the difference between the two reagents. Determination of the iron content of the reagent suggested that the microcirculation of an average brain contained some 1.8 g of whole blood. The finding that brain thromboplastin prepared from factor VII-deficient beagles is more sensitive to a deficiency of factor VII in plasma, presumably a result of the smaller quantity of factor VII present in the reagent, is compatible with the known kinetics of extrinsic coagulation.