Mammary gland tumors are a common tumor in the cat and most are highly aggressive tumors that will spread throughout the body. Treatment to date has included surgery with or without chemotherapy and/or radiation. However, these treatments have had disappointing results. A new drug, gefitinib (Iressa®), has been efficacious in human breast cancer clinical trials. Significantly fewer side effects occur with this drug than with chemotherapy or radiation therapy. Gefitinib may prove to be a readily available and effective treatment for feline mammary gland adenocarcinoma. In this study, researchers hope to show that gefitinib will be an effective treatment for breast cancer in the cat.