To explore the effect of beta-adrenoreceptor stimulation and blockade on the extraction of monosaccharide from the upper gut, we first established the malabsorption threshold in 26 normal volunteers using a series of test meals containing varying proportions of fructose and glucose. Incomplete small intestinal extraction and consequent arrival of carbohydrate into the caecum was identified by a rise in exhaled breath hydrogen concentration. The malabsorption threshold varied between individuals from 30 to 80 g fructose (median 40 g) but was reproducible within individuals, with 90% agreement of repeat studies. The malabsorption threshold for an individual was unrelated to body height (tau = 0.007, P > 0.05) or weight (tau = 0.003, P > 0.05) but correlated closely with time to onset of the breath hydrogen rise of a standard meal (tau = 0.70, P < 0.001). Administration of the beta-adrenoreceptor antagonist propranolol (160 mg) reduced the quantity of fructose required to exceed the malabsorption threshold from 45, 30-60 (median and range) to 40, 30-50 g (P = 0.03); administration of the beta-adrenoreceptor agonist isoprenaline (0.015 micrograms.kg/min) increased the quantity of fructose required to exceed the malabsorption threshold by 10 g (55 (50-90) g; P < 0.02). The effect of both drugs correlated closely with their transit effect (tau = 0.79, P < 0.01). A beta-adrenoreceptor mediated pathway thus appears to be capable of influencing the extraction of monosaccharide from the small intestine in normal subjects both under resting and stimulated conditions, probably acting via an effect on upper gastrointestinal motility.