October 7: 3 p. m., 6.5 cc 2 per cent caffein (0.232 gram per kilo) aqueous solution administered by subcutaneous injection; 3.35 p. m., reflexes increased. October 10: found dead. _Autopsy_: Nos. 70, 71, 72 showed congestion of organs. The reaction to caffein in the experiments of this series (C) showed considerable variation. The appearance of symptoms, as well as the final outcome of the experiments, differed markedly in a number of cases, notwithstanding the fact that the conditions were the same; thus the administration of 0.2 gram per kilo to guinea pigs, all of which received the same diet, induced no symptoms in two of the animals (Nos. 66 and 65), while marked symptoms were observed in the other four; in two of these the symptoms appeared in one hour and a quarter after injection, and in two others (Nos. 68 and 69), mild symptoms only appeared in 20 or 25 minutes. The last two were under observation for 4 hours longer, but there was no visible change in their condition. The duration of life in all of these guinea pigs, as indicated in the table, likewise varied. Two (Nos. 60 and 68) died during the night after they received caffein, one survived (No. 65), and three others (Nos. 57, 66, and 69) lived 2½, 5, and 9 days, respectively. Experiments with larger doses likewise showed differences in the behavior of these animals toward caffein, but they were not quite so marked. As shown in the table, symptoms appeared in from 35 minutes to 1.5 hours after injection. The duration of life was less than 1 day in two pigs, about twice as long in two others, and in one case between 2 and 3 days. A comparison made with results obtained in the preceding series shows a striking difference in the resistance to caffein. As 2 decigrams per kilo proved more rapidly fatal to the guinea pig than the larger doses employed in the later experiments, this difference in the resistance to caffein may be due to several factors. As pointed out in the experiments on rabbits, age might be an important factor influencing the toxicity of caffein. Unfortunately, no accurate data were available on the age of the guinea pigs, but they were all apparently full grown, although they differed in weight considerably. The difference in their ages was in all probability not very great. Moreover, it will be observed that the resistance in series B and C differed in animals of approximately the same weight. This is evident on comparing experiments Nos. 20, 38, and 13 of series B with Nos. 65, 66, and 69 of the next series. Again, further inspection and analysis of these tables show no difference in the toxicity, although there may be considerable difference in the weight, from which it may be concluded that the animals were of about the same age or that this plays no part in the resistance to caffein in the guinea pig.