Infection with Toxoplasma gondii in the acute phase results in nonspecific suppression of immunologic function in mice and humans. The present study examined the effects of a physical stressor, i.e., cold stress (CS), on macrophage function (nitrite production, parasite survival) and splenic blastogenesis in the acute phase of murine T. gondii infection. In our stress paradigm, female BALB/c mice were placed in cold water (1 +/- 0.5 C), 5 min each day for 8 days. Nitrite production and parasite survival were measured in cultured peritoneal macrophages obtained from mice subjected to CS after in vivo activation with interferon-gamma/lipopolysaccharide (CS + ACT), and in vitro infection with T. gondii tachyzoites. Peritoneal macrophages from CS + ACT mice showed decreased nitrite production compared to control but activated cells (ACT). Spleen cell proliferation to in vitro stimulation with the mitogens concanavalin A (Con A) and anti-CD3, and Toxoplasma lysate antigen (TLA) was measured in splenocytes obtained from BALB/c mice during the acute phase of infection with T. gondii. Mice subjected to CS and infection (CS + INF) had maximum splenocyte proliferation on days 8 and 15 followed by a subsequent decline on day 28 postinoculation (PI). In contrast, infected mice not subjected to stress (INF) showed decreased splenocyte proliferation on days 8 and 15 followed by an increase on day 28 PI. The rate of mortality was decreased in the CS + INF compared to the INF group during acute infection. These results suggest that CS may alter the pathogenesis of T. gondii infection by modulating acute-phase responses, provoking a state of transient disequilibrium between the host and parasite.