Growing pigs (initial BW 14.3 +/- 1.2 kg) were fed isocaloric (3.26 Mcal of ME/kg) and isonitrogenous (16% CP) diets containing either 0 (low fiber, LF; n = 4) or 10% (high fiber, HF; n = 4) wheat straw for ad libitum intake for 14 d. On d 14, each pig was injected i.v. with bromodeoxyuridine (BrdU, a thymidine analog; 5 mg/kg) and was slaughtered 1 h later. Visceral organs (liver, pancreas, and intestines) were weighed, and tissue samples were obtained. Feed consumption, daily gain, gain: feed, and final BW did not differ between treatments. Neither visceral weights nor visceral weights per unit of eviscerated BW were affected by diets. Tissue concentrations of DNA (milligrams/gram of tissue) were lower (P < .03) in HF than in LF only for jejunum, ileum, and liver. Contents of DNA and protein (milligrams) did not differ between LF and HF for intestinal segments or liver. Content of RNA (milligrams) was greater (P < .04) in HF than in LF only for colon. The number of crypt cell nuclei that were labeled with BrdU (indicating DNA synthesis and thus cell proliferation) was increased (P < .03) in HF relative to LF for jejunum and colon. The number of epithelial cells exhibiting DNA fragmentation (indicating programmed cell death) was greater (P < .07) in the HF than in the LF group for jejunum and ileum. Width of intestinal villi was increased (P < .10) in HF vs LF for jejunum and ileum. Depth of intestinal crypts was increased (P < .08) in HF vs LF for jejunum, ileum, and colon.(ABSTRACT TRUNCATED AT 250 WORDS)