These experiments were carried out on well-fed, full-grown gray rabbits. The diet for several days preceding the experiments consisted of oats or carrots, which were given ad libitum. The injections were made into the ear veins from a burette or by means of a syringe, the temperature of the caffein solution being about 40° C. Attention was also directed to the effect of the rate of injection and of the concentration on the toxicity. The minimum toxic as well as lethal doses were determined as shown in the following experiments. SERIES A. In these experiments the rate of injection was about 1 cc of 2 per cent caffein solution per minute. _Rabbit 194. White, female. Weight, 1,310 grams._ October 19: Injected 7.5 cc 2 per cent solution caffein (115 mg per kilo) into the ear vein. Rabbit showed stiffness; paralysis of extremities appeared soon after.[C] Rabbit survived. [C] Time of injection inadvertently omitted, but was probably not slower than in the other cases of this series. _Rabbit 556. Gray, female. Weight, 1,635 grams._ January 31: 2 p. m., 11 cc 2 per cent caffein (134 mg per kilo) injected into ear vein, in about 11 minutes; 2.10 p. m., convulsions, rabbit remained lying on its side; during the rest of the hour it had convulsions occasionally; 3.20 p. m., convulsions and died. Rabbit did not urinate after the injection of caffein. _Rabbit 557. Gray, female. Weight, 1,580 grams._ January 31: 2.30 to 2.37 p. m., 7 cc 2 per cent caffein injected from the burette at the rate of 1 cc per minute; 2.37 p. m., flow of liquid ceased, veins were engorged and bled freely, injection was continued by means of a syringe; 2 cc 2 per cent caffein injected in two minutes; injections discontinued as convulsions appeared; 2.50 p. m., rabbit raised itself but fell over; 3.10 p. m., rabbit assumed normal attitude, walked about the floor without manifesting any signs of the effects of caffein; 4.30 p. m., walked about, gait normal, condition seemed to be good. February 1: 2 p. m., condition good, appetite good, total amount of caffein injected, 9 cc 2 per cent solution, or 114 mg per kilo. _Rabbit 558. Gray, female. Weight, 1,590 grams._ January 31: 3 p. m., given 8 cc 2 per cent caffein in eight minutes; 3.10 p. m., violent convulsions; 3.20 p. m., rabbit was stretched out on his abdomen, extremities extended, urinated; 4.30 p. m., looked normal; was able to walk about. February 1: 2 p. m., condition good, appetite good. _Rabbit 292. Belgian hare, male. Weight, 1,770 grams._ February 18: 4.26 to 4.39 p. m., 12.5 cc warm caffein solution (0.141 gram per kilo) injected into ear vein, convulsion followed when this quantity was injected, tonic rigidity of limbs followed soon after; 4.52 p. m., condition unchanged, rabbit on floor, limbs stretched out, and lying on abdomen. _Rabbit 294. Belgian hare, female. Weight, 1,350 grams. Carrot diet for about 10 days before the experiment._