Background and objectives: The role of opioids on tumor growth is widely discussed. The aim of this study was to analyze whether morphine has a direct influence on tumor growth. Methods: Twenty mice received intraperitoneal inoculation of Ehrlich tumor cells and were divided into two groups. Morphine group received 10 mg/kg, and the control group received 0.9% saline solution, once daily for 7 days, by gavage of 10 ml/kg. On the tenth day, 5 animals were sacrificed, and 5 were evaluated for survival in each group. The following were evaluated: survival; abdominal circumference; ascites volume; weight; tumor cells in ascites; lymphocytes in the spleen, inguinal and mesenteric lymph nodes and bone marrow; cytokines in ascites, serum, lymph nodes and spleen; nitric oxide (NO) in ascites and spleen; nitrite in ascites; arginase in ascites; superoxide dismutase (SOD) in ascites; and immunophenotyping of the spleen. Results: The number of cells in the bone marrow was higher after morphine treatment; SOD was higher in ascites after morphine, and the NO level of the spleen stimulated with lipopolysaccharide (LPS) was lower after morphine treatment. There were no differences in the other parameters. Conclusion: The results of this study show that morphine causes some changes that facilitate tumor growth in mice as higher NO in ascites and, although there were no significant differences, weight, ascites volume and the abdominal circumference was higher after morphine treatment.