Lactulose is often used as a substrate to estimate the mouth-to-caecum transit time (MCTT), but because of osmotic effects the outcome depends on the dose consumed. In this study, barley groats, a complex carbohydrate (CH) that produces a clear breath hydrogen response after consumption, were used. Eight volunteers consumed the same dose of softened barley groats (1.0 g CH/kg body weight) three times. Ten volunteers ate three different doses (0.75, 1.0, and 1.5 g CH/kg body weight). Breath samples were collected every 30 or 60 min for at least 12.5 h and analysed for hydrogen. To study the effect of the particle size of the barley groats or the addition of fat on the MCTT, 11 volunteers consumed barley groats (1.0 g CH/kg body weight) with cream cheese (0.25 g fat/kg body weight) and 5 volunteers ate crushed barley groats (particles of +/- 1 mm, 1.0 g CH/kg body weight). After consumption of 1.0 g CH/kg body weight a mean MCTT of 8.4 +/- 0.4 h was found. After consumption of the high dose, a mean MCTT of 9.0 +/- 0.5 h was found, and after the low dose, 8.5 +/- 0.6 h. Addition of fat resulted in a significantly increased MCTT, presumably caused by retarded stomach emptying (9.2 +/- 0.6 h versus 8.1 +/- 0.5 h in controls). Particle size did not significantly affect the MCTT (7.2 +/- 0.5 h at 1 mm versus 7.8 +/- 0.6 h at normal particle size).(ABSTRACT TRUNCATED AT 250 WORDS)