Mongolian gerbils were susceptible to infection with Giardia lamblia cysts from patients. Inoculation of gerbils with 5 x 10(3) cysts each resulted in an infection characterized by the intermittent release of cysts for up to 39 days. The mean number of cysts released per gerbil in a 2-hr period was 8.8 x 10(2) (range, 0-5 x 10(3)). The highest number of trophozoites found in the intestine was on day 15 after infection, when the mean number of trophozoites per gerbil was 6.36 x 10(6). Administration of cysts from different patients to gerbils resulted in a similar pattern of cyst release during the first 30 days of infection. Mongolian gerbils were also susceptible to infection with cultured trophozoites (Portland 1 strain). The pattern of cyst release and the number of trophozoites in the intestines of orally and duodenally inoculated gerbils were similar. Gerbils were protected against reinfection with G. lamblia for up to eight months after primary infection.