1. The effect of total starvation for 4-5 days on the intestinal uptake and urinary excretion of markers from an orally administered mixture of mannitol (5g), [14C]mannitol (0.5 microCi), lactulose (10 g) and 51Cr-labelled ethylenediaminetetra-acetate (51Cr-EDTA) (30 microCi), was assessed in five lean (group 1) and four obese (group 2) subjects. The effect of a very low calorie diet for 1 week and of a subsequent 5 day period of total starvation on intestinal permeability was assessed in a similar way in another group of obese subjects (group 3). Transit time from mouth to caecum of the fastest component of the oral mixture was assessed by the appearance of hydrogen in breath (all subjects), and the configuration of the transit spectrum through various segments of the gastrointestinal tract, was assessed by a radionuclide scan method (group 2 subjects only). The effect of starvation on plasma/renal clearance of these markers in subjects of group 2 was assessed with the use of a bolus intravenous injection of a mixture of mannitol (2 g). [14C]mannitol (10 microCi), lactulose (0.1 g) and 51Cr-EDTA (5 microCi). 2. The uptake and urinary excretion of orally administered mannitol was decreased by total starvation. The mean decrease was 47% in the lean subjects (P less than 0.025), 33% in group 2 obese subjects (P less than 0.05) and 41% in group 3 obese subjects P greater than 0.05). In contrast, starvation produced no significant change in either the excretion of 51Cr-EDTA or lactulose.(ABSTRACT TRUNCATED AT 250 WORDS)