Forced jumping in the Lashley jumping apparatus, maintenance in cages with an electrified water source, transportation, sound-induced seizures, and electroconvulsive treatment were used in studying the effects of psychological stress on the prothrombin time of the albino rat. An increased prothrombin time occurred at certain times following forced jumping, sound-induced seizures, and electroshock. Frequently the altered coagulability was reflected in a greater heterogeneity of variance due to some animals having a longer prothrombin time and others shorter. The changes in the prothrombin time following stress were more pronounced when the prothrombopenic drug, Dicumarol, was administered. This was evident from two indices, prothrombin time and mortality.