The search of new therapies against cancer, both in human and in veterinary medicine, has lead to the development of a new group of drugs, named antiangiogenic drugs, due to their negative effect on the tumoral vasculature. The angiogenesis is defined as the development of new vessels starting from pre­existent vessels. Besides being present in cancer, where the genesis of new vessels is fundamental for the growth and dissemination to other tissues (metastasis), this phenomenon is also present in other diseases as well as in some physiologic processes.With the purpose of studying the effect of these drugs on the vascular bed of tumors, the effect of dexamethasone was analyzed in a canine transmissible venereal tumor (CTVT) of spontaneous presentation. Steroids have a proven negative activity on tumoral vessels. With this purpose, 7 dogs affected with CTVT were treated during 14 days with dexamethasone. During this time period measures of tumoral size were obtained, along with samples of blood and tumor biopsies. Tumor tissue samples were analyzed for vascularity by immunohistochemical analysis, using antibodies against von Willebrand factor as markers of endothelial cells. A significant decrease of the vascular area was observed in tumoral samples after dexamethasone treatment, in comparison with samples taken before treatment. This was clinically correlated with a small decrease in tumor size at the end of the treatment.-