A long-lasting, chronic infection with Toxoplasma gondii was established in C57Bl/6J mice as a model system of slow parasite infection. We quantified all nucleated cells and Thy-1+ and Thy-1- cell subpopulations in thymus, spleen and peripheral and mesenteric lymph nodes throughout the first 8 months of life of mice infected at 8 weeks of age. We found a physiological pattern of change with age in the lymphocyte subpopulations examined; the pattern was distinctive for each lymphoid organ. These normal patterns were altered in infected mice from the beginning of the observation period. The most prominent findings were (a) a relatively early atrophy of the thymus and (b) no increase in Thy-1+ lymphocyte numbers in either spleen or lymph nodes during the first 3 months post-infection, followed by decreasing numbers afterwards. Other findings were a rapid increase in the number of Thy-1- cells in the thymus before the onset of generalized atrophy; accumulation of erythrocytes in the spleen and splenomegaly during the first 3 months post-infection, despite a steady decrease in the number of nucleated cells; and an increase in Thy-1- cells in the peripheral but not in the mesenteric nodes, thereby diluting the Thy-1+ lymphocyte subpopulation.