Brain concentrations of dopamine, homovanillic acid, norepinephrine, serotonin and 5-hydroxy-indoleacetic acid were measured in mice with acute and chronic, adult-acquired toxoplasmosis. Mice with acute infections showed a 40% rise in homovanillic acid levels as compared with controls; dopamine levels, however, remained unchanged. Norepinephrine levels in this group were 28% lower than in controls. Dopamine levels were 14% higher in the mice with chronic infections than controls. Serotonin and 5-HIAA levels were not altered in infected mice. These neurochemical changes may be factors contributing to mental and motor abnormalities that accompany or follow toxoplasmosis in rodents and possibly in man.