Fewer adult Microphallus pygmaeus are recovered from the small intestine of adrenalectomized male mice than from sham-operated counterparts. Preinfection intraperitoneal injection of 2 IU or 4 IU ACTH daily for 7 days increases the initial primary worm burden in male mice. Preinfection intramuscular injection of 325 micrograms corticosterone daily for 7 days increases the initial worm burden and alters the subsequent rate of decline and duration of the primary infection. However, egg production is unaffected. The changes in parasite numbers are attributed to the immunosuppressive action of corticosterone.