Adult rabbits were inoculated orally (or duodenally) with virulent Vibrio cholerae O1. Jejunal colonization occurred only when hypoperistalsis was induced at the time of inoculation by tincture of opium given intraperitoneally (or by temporary ileal obstruction). For oral inoculation, prior neutralization of gastric acid was also required. Inoculation with 10(9) V. cholerae caused jejunal colonization for 1 to 2 days and ileal colonization for 5 to 6 days. The extent of small bowel colonization 18 h after oral inoculation was related to inoculum size but also reflected limited multiplication of small inoculum sizes and net death, clearance of large inoculum sizes, or both. Serious diarrhea occurred only in rabbits fed large inoculum sizes, i.e., 10(10) V. cholerae, and then rarely. Rabbits colonized once with 10(10) V. cholerae became highly resistant to recolonization with either the same or opposite serotype. After 18 weeks, these rabbits were still partially protected, whereas twice-colonized rabbits were highly protected. Protection against recolonization appeared to be due, at least partly, to interference with the adherence of V. cholerae to the bowel mucosa, thus allowing rapid removal of V. cholerae when peristalsis resumed. Prior colonization also protected against cholera-like diarrhea in rabbits challenged by the removable intestinal tie-adult rabbit diarrhea technique, the 50% effective dose for severe or lethal diarrhea being increased more than 100-fold, and probably more than 10,000-fold, for challenge with either the homologous or heterologous serotype of V. cholerae. The described rabbit model appears well suited for the study of immunity evoked by enteric colonization with V. cholerae O1.