Background: The cell mediated immunity has revealed the importance of activated macrophages as key immune effector cells. Activated macrophages have the ability to generate reactive oxygen and nitrogen species (ROS and RNS) to kill parasite. Objectives: The current study aimed to investigate the progression of nitric oxide (NO) and superoxide anion (O2-) production in activated macrophages, isolated from Schistosoma Mansoni-infected mice during innate immune response. Methods: Hundred male albino mice were divided into two main groups; control and infected. The mice of the infected group were injected subcutaneously via the tail with one hundred cercariae /mouse. In vivo production of NO and the O2- were estimated in the isolated peritoneal macrophages with the progression of post infection time. Results: The number of the isolated peritoneal macrophages and the production of NO and O2- were significantly increased exponentially with time intervals throughout eight weeks of infection compared to the control. The highest levels of both the peritoneal macrophages and NO as well as the lowest O2- value were shown at the 8th week post infection. The correlation analysis showed significant positive relationship between the number of peritoneal macrophages and NO production and non-significant positive correlation between their numbers and O2- production. In contrast, the NO and O2- production showed significant negative correlation in the activated cells. Conclusion: The activated macrophages are important immune effector cells that are capable of generating cytotoxic molecules such as NO and O2- during the prepatent period of Schistosoma Mansoni infection. However, the NO plays the key role in this innate immune response..