In this procedure a glass cannula was fastened in one of the external jugular veins and filled with the same solution as that to be injected. A short rubber tube was attached and tightly clamped close to the glass. Later, for the injection, the syringe needle was inserted through the rubber and into the fluid in the cannula, the clip on the vein was removed, and the injection made. The solutions employed intravenously were adrenin 1:10,000, 1:50,000, and 1:100,000, in distilled water. The smallest amount which produced any change in clotting time was 0.1 cubic centimeter of a dilution of 1:100,000 in a cat weighing two kilos, a dose of 0.0005 milligram per kilo. Four tests previous to the injection averaged 5 minutes, and none was shorter than 4 minutes. Immediately after the injection the time was 2 minutes, but at the next test the effect had disappeared. Doubling the dose in the same cat--i. e., giving 0.2 cubic centimeter (0.001 milligram per kilo)--shortened the coagulation time for about 40 minutes: Dec. 23. 10.30--4 minutes .35--4 “ .41--4 “ .46 Adrenin, 0.001 milligram per kilo. .47--2.5 minutes .50--3 “ .53--3.5 “ 11.00--1.5 “ .05--1.5 “ .10--3 “ .15--2 “ .20--4 “ .26--4.5 “ .31--5 “ From 10.47, immediately after the second injection, till 11.20 the average time for clotting was 2.5 minutes, whereas both before and after this period the time was 4 minutes or longer. At 11.00 o’clock and 11.05, when the end point was reached in 1.5 minutes (a reduction of 63 per cent), a thick jelly was found on examining the cannula. The changes in clotting time in this case are represented graphically in Fig. 26. [Illustration: Figure 26.--Shortening of coagulation time after injection of adrenin, 0.2 cubic centimeter, 1:100,000, (0.001 milligram per kilo), at 10:46. In this and following Figures a scale for coagulation time is given in minutes at the left.] In another case a dose of 0.0005 milligram per kilo failed to produce any change, but 0.001 milligram per kilo (0.28 cubic centimeter of adrenin, 1:100,000, given a cat weighing 2.8 kilos) brought a sharp decline in the record, as follows: Jan. 9. 11.32--6 minutes .40--6 “ .47 Adrenin, 0.001 milligram per kilo. .48--5.5 minutes .55--4 “ 12.00--5.5 “ .06--7 “ In these instances the animals were decerebrated. For decerebrate cats, the least amount of adrenin, intravenously, needed to produce shortening of coagulation time is approximately 0.001 milligram per kilo.