The behavior of mast cells was studied during tumor growth. Sarcoma 13, and normal syngeneic kidney, as control, were implanted subcutaneously in BALBc mice. The number of mast cells in the hypodermis and peritumoral tissue increased 3-fold, 20 days after implantation. In the peritumoral tissue, mast cell degranulation increased together with tumor growth, while in the dermis and hypodermis, it first decreased and only became evident on day 20. Mitosis and mast cell degranulation, more conspicuous in tissues near the tumor, seem to indicate the existence of tumoral factors which spread slowly from the tumor to distant zones. The role of mast cells in peritumoral tissues will be evaluated in the near future.