Sprague-Dawley rats were orally inoculated with Giardia lamblia cysts. Infection was documented using microscopic examination of feces, counterimmunoelectrophoresis (CIE) of feces for G. lamblia antigen, and intestinal biopsy. After inoculation with 150 cysts, 100% of the rats became infected. CIE and intestinal biopsy detected infection on days 2-3 after inoculation. Results of microscopic examinations were variable, but cysts were observed as early as day 4. CIE and intestinal biopsy were comparable in detecting infection. The infection rate peaked at day 7 and remained high until day 20. Spontaneous resolution of infection occurred after 28-42 days in 96% of the rats. Cysts stored in liquefied feces at 4 C demonstrated a gradual decline of infectivity but remained infective for one year.