1. The relationship between clotting time and heparin dosage has been studied in the dog.

2. On the addition of heparin to blood in vitro, a linear relation is found between heparin dosage and the logarithm of the clotting time obtained. The sensitivity of the blood sample to the action of added heparin is influenced both by the individual (coagulability of the blood before withdrawal) and by the technics of withdrawal and of determination of the clotting time. It is indicated that alterations in the latter may be used to extend the range of measurable hypocoagulability due to heparin. Incubation of heparin with blood for ten minutes increases its anticoagulant effect.

3. When moderate doses of heparin are injected intravenously, five to fifteen minutes are required for the clotting time to reach a maximum. No evidence of a biphasic response was obtained. The maximum clotting time obtained is greater than it is with the same amount of heparin added to the blood in vitro, due to the effect of incubation of heparin with blood on its anticoagulant activity. The in- terval required for the clotting time to return to normal is quite short, and with a given dosage is constant with different animals. Factors influencing the relation between duration of hypocoagulability and dosage are discussed.

4. A test has been devised to determine the sensitivity of the animal to the anticoagulant action of heparin. The clotting time response to certain concentrations of heparin added to the blood in vitro is determined. A fixed dose of heparin is then injected intravenously and the clotting time response is again determined. The response in vitro measures the sensitivity of the clotting system to heparin, while the in vivo response, when interpreted in the light of the in vitro response, measures the ability of the body to remove heparin from the circulation.

5. By means of this test, it has been determined that anesthesia with pentobarbital decreased the coagulability of the blood, urethane had no effect on coagulability, while the effect of ether was variable. The injection of india ink and evisceration caused a hypercoagulability, while removal of the kidneys had little effect.

6. When the sensitivity of the blood to the anticoagulant action of heparin was tested during these procedures, pentobarbital and nephrectomy had no effect, ether caused an increase in sensitivity, urethane a decrease. The injection of india ink and also evisceration markedly decreased the sensitivity of the blood to the anticoagulant action of heparin.

7. Anesthesia with pentobarbital, ether or urethane, the injection of india ink, removal of the kidneys, or removal of the gastrointestinal tract, had no effect on the duration of heparin action in the body.

ACKNOWLEDGMENT We are greatly indebted to Professor C. H. Best for his interest and encouragement in these studies, and to Dr. J. Markowitz for performing the experimental surgery for us. The study was supported by a grant from the John and Mary R. Markle Foundation.