Cofactors such as stress have been suspected to play a role in the susceptibility to opportunistic infections. Toxoplasma gondii is one of the major opportunistic infectious agents in immunocompromised individuals, and infection can be modulated by external factors such as stress.The purpose of this study was to examine the in vivo and in vitro role of cold stress (CS) in the pathogenesis of T. gondii infection and its impact on regulatory cytokines in this model.Mice subjected to CS and control animals were infected intraperitoneally with an LD(50) of PD2 T. gondii tachyzoites, and the outcome of the infection was determined. In addition, peritoneal macrophages obtained from CS and non-stressed mice were infected in vitro with T. gondii. The number of infected macrophages, the number of intracellular parasites and the production of interferon (IFN)-gamma, interleukin (IL)-12, and tumor necrosis factor (TNF)-alpha were determined.CS applied before intraperitoneal inoculation increased susceptibility against T. gondii infection. Peritoneal cells from CS mice contained significantly higher numbers of intracellular parasites and infected macrophages compared to those from non-stressed animals. IFN-gamma production was initially high in the CS group but decreased significantly after 36 h. Opposite results were found in the non-stressed group. Macrophages from CS mice persistently produced high levels of TNF-alpha and IL-12 and peaked after 36 h. Levels of these cytokines were lower or absent in the non-stressed group.These results suggest that CS increased the host susceptibility to intraperitoneal T. gondii infection by modulating the function of macrophages and the production of cytokines (IFN-gamma) involved in the early control of infection.