Synthetic antioxidants: EPO (p-ethoxyphenol), PHP (p-hydroxypropiophenone), and DPPD (N,N'-diphenyl-p-phenylenediamine) dissolved in dimethylsulfoxide were administered subcutaneously to mice after subcutaneous implantation of Lewis lung carcinoma or amelanotic B16 melanoma into the tails. The doses of antioxidants used were: 50 mg/kg, 100 mg/kg, and 200 mg/kg per day, respectively. The animals received five injections a week during two weeks. The tails with Lewis lung carcinoma were amputated after 16 days and with amelanotic melanoma after 20 days; tumors weight was measured. 25 days after Lewis lung carcinoma and 42 days after B16 melanoma implantation mice were sacrificed, lungs weighed and incidence of pulmonary and extrapulmonary metastases were estimated. There was a significant increase of weight of amelanotic melanoma in animals treated with EOP or PHP (p less than or equal to 0.05). No influence of antioxidants on the metastases incidence of B16 melanoma was found. Antioxidants used did not influence the Lewis lung carcinoma weight, whereas EOP or PHP increased the incidence of extrapulmonary metastases (p = 0.05, and p = 0.1, respectively).