It is assumed that the outcome of the lactulose/rhamnose gut permeability test is not influenced by pre- or post-absorptive factors. The aim of our study was to investigate the role of a pre-absorptive factor, i.e. small-intestinal transit, and a post-absorptive factor, i.e. renal clearance. Ten healthy male subjects were studied. Urinary lactulose and rhamnose excretion was measured after intraduodenal administration of lactulose and rhamnose following induction of increased intestinal permeability using chenodeoxycholic acid (chenodiol), in the absence and in the presence of accelerated intestinal transit. Urinary sugar excretion was measured after intravenous administration of either a regular dose (50 mg/50 mg) or a high dose (250 mg/250 mg) of lactulose/rhamnose. The intraduodenal experiments showed that a combination of accelerated small-bowel transit and increased permeability did not lead to significant differences in the recovery of lactulose (P=0.647) or rhamnose (P=0.889), or in the lactulose/rhamnose ratio, compared with those under conditions of increased permeability alone (P=0.68). However, lactulose recovery was significantly lower (P=0.025) after intravenous administration of a high dose of the sugars. There was no significant difference in urinary rhamnose recovery (P=0.575) between the high and the regular doses. This resulted in a significantly lower lactulose/rhamnose ratio (P=0.021) after intravenous administration of a high dose, compared with a regular dose, of the sugars. In conclusion, the assumption that post-absorptive processes do not influence the outcome of the lactulose/rhamnose permeability test appears not to be valid.