When mice were infected with Trypanosoma musculi during the second week of pregnancy, the level of parasitemia was significantly higher than in controls. Progesterone and estradiol injected daily into normal virgin mice during the course of infection did not modify the parasitemia. Daily injections of low-dose hydrocortisone into infected mice produced elevated parasitemias although these never reached the levels obtained during pregnancy. It is postulated that systemic levels of gestational hormones are only a minor contributing factor for elevated parasitemia.