Neovascularization and tumor vessels in various cancers transplanted into a transparent chamber in rat skin were investigated. The following results were obtained: 1) Development of neovascularization toward an avascular area was apparent one week after the installation of the chamber. 2) Differentiation into arterioles and venules was a common occurrence. Arteriolar neovascularization were more vigorous toward an implanted tumor than toward an avascular area. 3) The change of pre-existing normal vessels into tumorous one was characteristic. Vessels within tumors became abnormal in size, dilatated, tortous, and showed cross-anastomosing and distortion at the capillary level. Slowing of blood flow was marked in tumor vessels. This abundant vascular bed seemed inefficient mechanically as well as functionally, as tumor tissues became hypoxic. The tumor developed necrotic centers so that, finally, tumor cells survived only within narrow space around large vessels.