In order to assess the effects of a latent Toxoplasma infection on behaviour and learning capacity, maze experiments with rats and mice were performed. The learning performance of rats and especially of mice was impaired, the memory of mice and their overall activity were reduced by infection. Theses results are supported by Hutchinson who also found that the activity of mice was reduced by Toxoplasma infection, and that they were less responsive to novel stimuli. These results indicate that Toxoplasma infection probably affects the animal's response to its environment, and the stimulation arising from it, and may even affect endogenous regulatory processes in the brain. Similar observations in humans suggest that an acquired Toxoplasma infection without giving rise to disease may have similar effects in man. These observations touch the general problem of the influence of parasitic infections on the CNS.