Canine hemangiosarcoma (HSA) is an aggressive cancer of vascular endothelial origin with the standard of care treatment for splenic HSA resulting in published median survival times ranging from 1-3 months with splenectomy alone and 4-6 months with adjuvant doxorubicin-based chemotherapy; less than 10% of dogs are alive 12 months after diagnosis. Given this poor prognosis, investigation of additional therapies for canine HSA is needed. The combination of dacarbazine-doxorubicin adjuvant chemotherapy improved upon these survival outcomes in a recent study, but the clinical use of dacarbazine has been limited given the logistics of its administration and potential for toxicity. Dacarbazine is administered intravenously as a long infusion over 8 hours or as a bolus by slow IV injection once daily for 5 consecutive days every 3 weeks; both long infusions and intensive daily intravenous chemotherapy protocols pose logistical and financial challenges for pet owners and veterinarians, as well as increased risk to the dog receiving the drug as accidental extravasation may cause severe local irritation and pain. In addition, dacarbazine has been associated with a clinically significant and dose-limiting side effect profile including gastrointestinal tract toxicosis such as nausea, vomiting, and diarrhea, and severe to life-threatening hematologic toxicosis, including neutropenia and thrombocytopenia. Temozolomide, a drug closely related to dacarbazine, appears to be better-tolerated in dogs and its dosing route and schedule is more convenient; owners are able to administer the drug orally at home. In the proposed pilot study, dogs with splenic hemangiosarcoma will be prospectively enrolled to receive adjuvant doxorubicin chemotherapy in combination with the study drug temozolomide. The benefit to dogs will be evaluated by assessing overall survival, progression free survival, and drug tolerability. For comparison we will look at the outcomes and side effects experienced by historical control dogs treated for splenic HSA previously at Cornell University with only adjuvant doxorubicin chemotherapy. This study will be available for enrollment through the Cornell University College of Veterinary Medicine Oncology Service.