After intragastric inoculation of adult mice, type 1 reovirus was initially concentrated in Peyer's patches over the first 4 hr after inoculation, then spread sequentially to the mesenteric lymph nodes and spleen. For type 3 reovirus, however, initial entry into Peyer's patches in adult mice was followed by loss of viral infectivity so that by 4 hr after inoculation virtually no infectious virus was detected in the intestine, and spread to extraintestinal tissues did not occur. In 10-day-old mice, type 3 was capable of spread to the mesenteric lymph nodes but not the spleen. Thus, as animals aged there was a greater restriction of the spread of type 3 from the intestine. Studies using a field isolate of type 3 reovirus that is resistant to intestinal proteases, and genetic studies utilizing type 1 × type 3 viral reassortants, revealed that the viral σ1 protein determined the capacity of reovirus to spread from the intestine in both adult and 10-day-old mice. Thus, the interaction of reovirus with host defense mechanisms, and the age-dependent restriction of spread of type 3 reovirus from the intestine are mediated by the viral σ1 protein.