This study investigated the relationship between bacterial translocation (BT) and small and large bowel bacterial colonization in the neonatal rabbit. The authors have previously shown that spontaneous BT occurs in rabbits during the first week of life, but is less frequent before the third day of life. The authors have also shown that bacterial colonization of the small bowel, is delayed until 3 to 5 days of age, and is preceded by colonization of the large bowel. Therefore, the authors hypothesizes that BT is dependent on small bowel, not large bowel, colonization. New Zealand White rabbit pups (n = 255) were divided into four groups, formula-fed (FF), formula-fed plus antibiotic (FFAB), breast-fed (BF) and unfed controls (UC). Animals from each group were killed on day 1 to 4 of life. Small bowel (SB) and large bowel (LB) specimens, mesenteric lymph nodes (MLN), spleen (SPL), and liver (LIV) were obtained from each rabbit, incubated for 24 hours in thioglycolate broth, and plated on both MacConkey and Colistin Naladixic Acid media in an aerobic environment. After 24 hours, growth on either plate was recorded. Bacterial colonization was significantly greater in FF versus UC, BF, and FFAB rabbit groups. In addition, large bowel colonization was significantly increased compared with small bowel colonization in all feeding groups (P < .05). Bacterial translocation to MLN, SPL, and LIV ranged from 10% to 27% in all groups. Gram-negative bacteria predominated over gram-positive bacteria in translocation to the various tissues sampled.(ABSTRACT TRUNCATED AT 250 WORDS)