The proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) is one of the main mediators of neuroimmune interactions. Increased production of TNFalpha in nerve and immune system cells occurs in infectious diseases, traumas, ischemia, and hypoxia. The present study addressed the role of TNFalpha in the formation of behavior during early postnatal ontogenesis. TNFalpha was given to rats during the third week of life and behavior was tested at ages 1.5 and 3 months using the open field test and the elevated plus maze test. Experimental rats differed from controls in terms of changes in orientational behavior on testing at 1.5 months and an increase in anxiety on testing at 1.5 and 3 months. The different roles of proinflammatory cytokines in the brain at the early stages of ontogenesis are discussed.