1. Diphtheria toxin of known strength, injected intravenously into rabbits in single doses, produces lesions which are first evident in the endothelium of the tufts, and possibly in the intertubular capillaries.
2. The lesions of greatest severity with medium doses are also found in the tufts.
3. Evidence is presented to show that epithelial damage in the tufts and especially in the tubules is secondary to the vascular injury.
4. The severity of the lesions was roughly proportional to the amount of toxin injected. Hemorrhages did not occur with doses of less than 0.0023 cc. of toxin per kilo of body weight and were not extensive with doses of less than 0.004 cc. per kilo.
5. It is suggested that the localization of lesions in the tufts is due to the concentration of toxin in the blood that follows the loss of water at this point.
6. It is further suggested that this inference may be extended to explain the pathogenesis of the so called intracapillary form of glomerulonephritis.