The effect of a 22 day space flight on the lymphoid organs of rats was investigated. The flight leds to hypoplasia of lymphoid organs, the spleen and thymus weight decreasing very noticeably. Histological, morphometric and cytological examinations demonstrated that splenic hypoplasia occurred due to a reduction of the number of lymphocytes and erythroid cells and hypoplasia of the thymus and lymph nodes due to a decrease of the lymphocyte number. Changes found in the lymphoid organs of the flight rats were reversible and the structure of lymphoid organs recovered 27 days postflight. Pathogenetic mechanisms of the changes developing in lymphoid organs of rats under the influence of space flight were discussed. The animal data were compared with the results of postflight medical examinations of astronauts.