Rats were infected by intragastric inoculation of Yersinia enterocolitica 0:3 grown at room temperature. The events of the first 6 days of the infection were followed by staining sections of small intestine using an immunoperoxidase method. The specific antibodies used were either rabbit antibodies to antigens of Y. enterocolitica, to the temperature-inducible antigens specified by the virulence plasmid (pYV) of Y. enterocolitica, or monoclonal antibody to the pYV-specified autoagglutination protein P1. In the course of the infection Y. enterocolitica organisms were detectable in the terminal ileum already 1 h after the challenge. By 1 h the pYV-specified temperature-inducible antigens were expressed in Y. enterocolitica both in the lumen of the intestine and in the intestinal tissues, which indicates that the activation of pYV in vivo was very rapid. Y. enterocolitica organisms which had penetrated into the lamina propria of the villi were rapidly removed by phagocytic cells. Later organisms were located intracellularly in the lamina propria, in Peyer's patches and in the regional lymph nodes. The terminal ileum was the most severely affected part of the small intestine.