_Guinea pig 20. Female. Weight, 497 grams. Diet, oats._ April 2: 5 cc 2 per cent caffein injected subcutaneously at 11.30 a. m.; 1.50 p. m., spasm of short duration. Died at 3 p. m., three and one-half hours after injection. _Guinea pig 38. Brown male. Weight, 570 grams. Diet, carrots and oats week previous to injection._ February 11: 3.50 p. m., 6 cc 2 per cent caffein injected subcutaneously in back (210 mg per kilo); 4.15, reflexes increased, had convulsion of short duration when disturbed; 4.45 p. m., on handling, repeated convulsion and paralysis; 5 p. m., guinea pig lying on his side, respiration difficult and labored. February 11: 5.05 p. m., guinea pig found dead, 2 hours and 15 minutes after injection. _Guinea pig 37. Male. Weight, 820 grams. Diet, carrots and oats during week preceding the injection of caffein._ February 11: 3.35 p. m., 8.5 cc 2 per cent caffein injected subcutaneously in the back; 5 p. m., pig very sensitive, anterior extremities paralyzed when handled, frequent spasms of posterior extremities, no symptoms noticed before 5 p. m., although watched all the time; 5.05 p. m., guinea pig on his legs and looked normal. No attack on handling. February 12: 9 a. m., found dead; died within 18 hours. _Guinea pig 13. Female. Weight, 618 grams. Diet, oats._ March 29: 2.45, 6 cc 2 per cent caffein injected subcutaneously (0.194 grams per kilo). March 30: Died at 4 p. m., 25 hours after injection. _Guinea pig 36. Male. Weight, 850 grams. Fed oats and carrots for one week previous to injection._ February 11: 3.30 p. m., 8.5 cc 2 per cent caffein injected subcutaneously into back; 5 p. m., somewhat more sensitive than normal, no other symptoms, no effect on handling; 5.05 p. m., no symptoms. February 12: 9 a. m., found dead, about 18 hours after injection. The results of these experiments, as observed in five guinea pigs, indicate that two decigrams of caffein per kilo of animal produce symptoms within a half to about two and a quarter hours after injection. Death followed in two guinea pigs 70 minutes to 1 hour after the first manifestations of symptoms. Two others died during the night, while one lived 25 hours after the injection of caffein. Even 2 decigrams caffein per kilo weight might therefore be fatal to the guinea pig. Experiments carried out later have shown, however, that the resistance to caffein is appreciably greater in some guinea pigs. This is indicated by the following experiments, in which doses of 0.2 to 0.24 gram caffein per kilo were administered by the same path. SERIES C. _Guinea pig 66. Yellow and dark brown male. Weight, 510 grams. Diet, oats._ October 4: 5 cc 2 per cent caffein (0.2 gram per kilo) injected subcutaneously in the back at 3 p. m.; 5 p. m., no symptoms. October 5: 9 a. m., alive; condition good. October 9: Found dead. _Autopsy_: Congestion of liver, kidney, and small intestine. _Guinea pig 65. White and black male. Weight, 510 grams. Diet, oats._