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Dehiscence of anastomosis in the large bowel (LB) is more frequent compared with that in the small bowel (SB). The pathophysiological mechanisms underlying the phenomenon are well known. The goal of this study is to demonstrate the different oxidative phosphorylation roles in the response of the anastomosed SB and LB. In 45 male rabbits under general anaesthesia, the ileum and colon were transected always on the same side and an end-to-end anastomosis was constructed using single-layer inverting interrupted 5-0 Ethilon sutures. Tracts containing bowel anastomoses were collected at 3, 12 and 24 h and 3, 7 and 21 days after the operation and compared with non-anastomosed bowel. Respiratory rates with different substrates, enzymatic activities, respiratory complex structures and cytochrome contents were measured in smooth muscle purified mitochondria. Mitochondrial respiration showed significant differences in the LB versus anastomosed SB: respiratory rates were markedly reduced in the LB (residual activity: 25 vs. 95% of the SB). Detailed analysis of other mitochondrial parameters confirmed a better tolerance of the ileum to anastomosis. From these preliminary studies we can consider two different aerobic thresholds for either the large or the small rabbit intestine, the latter being oxygen dependent. This can explain the mitochondrial sufferance of anastomosis in the LB whose prevalent aerobic metabolism can be more easily affected by different noxae if compared with the tendency to glycolytic metabolism of the small intestine. [ABSTRACT FROM AUTHOR]