Vascular re-action to traumatic injury, homologous transplants of nor-mal tissues, and to transplanted sarcomas and mammary-gland carcinomas was studied in strains C3H and C mice. Transparent chambers were inserted into skin flaps and a quantitative technique was used for the measurement of vascular reaction. In wound healing .... the following sequence of events was observed: Accumulation of fluid and leukocytes at the site; capillary dilation in adjacent areas; new capillary formation after a latent period of approx. 6 days; rapid proliferation of new vessels during next few days, attaining a peak which was apparently correlated with the amt. of fluid and cellular reaction to the injury; differentiation of some of the newly formed capil-laries into venules and arterioles; rise of vascular level from 0 until it equalled or surpassed that of the surrounding tissues, then receded and stabilized at that level. Vascular reaction in surrounding tissues subsided to connective-tissue level at stabilization. In transplanted sarcomas and mammary-gland carcinomas, the vessels originated from host as capillaries.....Differentiation into arterioles and venules was not evident. An outstanding characteristic of the tumor cell was its capacity to elicit continued growth of new capillary endothelium from the host.