Abnormal platelet counts have been observed in patients with iron deficiency anemia. To study this relationship, rats were made iron deficient, and platelet production was estimated by radiosulfate incorporation into platelets. With progressive iron deficiency anemia, both platelet counts and the rate of platelet production increased significantly. Platelet survival was normal. Injection of iron was followed by a fall in platelet counts and platelet production to normal levels within 72 hr, at which time stainable iron had appeared in the bone marrow. An inverse relationship between platelet counts and hematocrit was also seen. It appears that platelet homeostasis in iron deficiency anemia is influenced by the duration and severity of anemia.