It has been suggested that the parasite Toxoplasma gondii reduces the fear of rodents toward their feline predators, which may lead to an augmented rate of predation and multiplication of the parasite through an increased number of life cycles. To investigate whether T. gondii infection induces selective effects on behavior associated with anxiety, Wistar rats were inoculated i.p. with several doses of T. gondii tachyzoites and tested in two animal tests of anxiety. In the third week following inoculation, rats infected with 100 and 1000 tachyzoites increased plus-maze open arm exploration in a dose-related manner. However, no effect was detected in either social interaction levels or motor activity measures. In the seventh week after inoculation, rats infected with 100 and 1000 tachyzoites showed increased open arm exploration and social investigation without change on any motor activity measures. However, rats infected with a higher dose (1500 tachyzoites) showed a drop in locomotion. These data support the hypothesis that T. gondii impairs mechanism of warning as a function of reduced anxiety. The pattern of brain colonization by the parasite and the host immune response suggests that the predominant invasion to limbic areas works as a natural anxiolytic mechanism.