A new transparent tissue chamber devised for in vivo microscopic study of the hamster cheek-pouch membrane is described. The morphology of the microcirculation in 4 transplantable hamster tumors has been studied in grafts made into the chamber. Grafts from several transplant generations of 2 pigmented melanomas that had the same vascular pattern, an anaplastic carcinoma of the breast, and a hemangiopericytoma without exception reproduced vascular patterns characteristic for each type of tumor. The 3 different patterns observed were distinctive and typical examples are illustrated. The possible ontogenetic significance of the characteristic anatomy of the microcirculation in these tumors is discussed.