The effects of gender and psychoneuroimmunological factors resulting from the social environment and status of males were investigated with regard to the concentrations of testosterone and corticosterone and the course of Trypanosoma cruzi infection in mice. Hormone concentrations varied considerably; and only testosterone concentrations showed a tendency to be higher in dominant males. Females kept singly developed lower and more similar parasitaemias than males kept singly or together with a female. This difference was significant when comparing groups of females or males. Within groups of male mice, parasitaemia was strongly correlated with the social position, being high in inferior males and low in dominant ones. The importance of these findings is that they clearly prove that chronic social stress in males strongly affects the course of infection with T. cruzi.