1. Young splenectomized female rats, free of latent infection, show greater increases in ovarian weight in response to injections of pregnant mare's serum than do young normal rats with spleens intact. 2. The regression in ovarian weight which occurs after about a month in such injected splenectomized animals may be prevented by repeated injections of an agent like trypan blue which causes blockage of the compensating reticulo-endothelial elements. 3. The plasmas obtained from these splenectomized and splenectomized dye-blocked animals possess less antihormone substance than plasmas from similarly treated normal animals. 4. Young female rats, heavily infected with Bartonella muris, and therefore possessing an injured reticulo-endothelial system, develop heavier ovaries in response to injections of pregnant mare's serum than do normal uninfected rats. 5. Similar differences in effect on the testes and seminal vesicles of young normal, splenectomized, and bartonella-infected male rats have been obtained using pregnancy urine extract. 6. Young splenectomized, and splenectomized dye-blocked guinea pigs injected with thyrotropic extract, show heavier and more highly active thyroids than normal hormone-injected animals, 7. These results are explained on the basis that the reticulo-endothelial system participates in the production of antihormone substances.