February 13: 1 p. m., still alive. February 14: 9 a. m., found dead. _Guinea pig 45. Female. Weight, 435 grams. Diet, oats for about one month previous to injection._ March 17: 3 p. m., 3.5 cc of 2 per cent caffein injected subcutaneously in the back (0.160 gram per kilo); 4.35 p. m., no symptoms; 5.40 p. m., no symptoms. _Guinea pig 39. Male. Weight, 820 grams. Diet, oats and carrots._ February 12: 11 a. m., 6 cc (0.15 gram per kilo) 2 per cent caffein injected subcutaneously in back. February 14: 9 a. m., alive; seemed to be in good condition; found dead at 1 p. m. _Guinea pig 41. Weight, 660 grams. Diet, oats and carrots one week previous to injection._ February 12: 11 a. m., 5 cc (0.15 gram per kilo) 2 per cent caffein injected subcutaneously. February 14: 2 p. m., pig alive; apparently normal. February 18: Guinea pig still alive and apparently in good condition. _Guinea pig 46. Female. Weight, 470 grams. Diet, oats about one month previous to experiment._ March 17: 3.15 p. m., 4 cc (0.170 gram per kilo) 2 per cent caffein injected into back subcutaneously; 4.35 p. m., reflexes increased, tremors on handling marked; 5.40 p. m., no change, symptoms about as before. March 18: 2.30 p. m., no symptoms. The experiments of this series (D) likewise showed a considerable difference in the resistance of the individual guinea pigs. Nos. 41, 45, and 46 survived; the rest of the pigs died within 18 hours to 2 days after the administration of caffein. Since an autopsy was held on one only, it is impossible to assign a cause for the variation in the toxicity of caffein in these guinea gigs, as the diet and the other conditions under which the experiments were conducted were the same. It was found in the experiments on cats and rabbits that the presence of morbid processes tends to increase the toxicity of caffein. The observations of Ophüls(66) are of interest in this connection. He found spontaneous lesions of the kidney and liver in a large proportion of guinea pigs examined. The greater susceptibility to caffein of guinea pigs Nos. 39, 40, 49, is probably due therefore to some pathological change which increased its toxicity. About 0.2 to 0.24 gram per kilo may therefore be regarded as the minimum lethal dose for the normal guinea pig when caffein is introduced subcutaneously, the minimum toxic dose being about 150-160 mg per kilo.