We have recently reported a dietary method for the production of thrombosis and infarcts in rats and have suggested that part of the mechanism of thrombosis in these animals was interference by the diet with some hematologic factor, possibly fibrinolysis. In the study reported herein, we have developed a method for studying fibrinolysis in rats in vitro, utilizing an electronic device called the Thrombelastograph. Results of this study correlate well with the previously demonstirated thrombogenic properties of 2 of the diets. Rats on a “butter-thiouracil-cholesterol-bile salt” diet (known to produce thrombi and infarcts) have prolonged clotlysis times, as compared with rats on a similar diet (and not thus far associated with thrombi and infarcts) in which corn oil is substituted for butter. Although the in vitro test is performed under highly artificial conditions, they are carefully standardized and the fact that the results correlate well with previously demonstrated thrombogenic properties of the diet would suggest that it has practical value for the study of thrombogenic effects of various diets in experimental animals. It is possible that it will prove to be even more useful in the study of fibrinolysis in man.