The influence of extensive thermal on selected functions of the reticuloendothelial system (RES) was studied in 35 rabbits of both sexes. Phagocytic activity was evaluated by the method of Malpern et al. and by the isotope method. The results indicate marked prolongation of the depression of phagocytic function of the RES in the course of burn disease. In animals which died early in the course of burn disease, RES activity was depressed throughout the period of observation. In animals which survived burn disease, RES activity underwent periodic changes.