The effects of hydroxypropylmethylcellulose on upper gastrointestinal transit, viscosity, and water flux were studied in six dogs fistulated at the proximal duodenum and/or mid-jejunum. Combinations of different grades of hydroxypropylmethylcellulose were prepared as 2% or 3.3% solutions to yield input viscosities of low (approximately 5000 cp at 37 degrees C and 1 s-1), medium (15,000 cp), or high (30,000 cp) viscosity. Hydroxypropylmethylcellulose modified intralumenal viscosity, with a linear relationship existing between input and lumenal viscosity. With regard to transit, the lag time before the onset of chyme recovery increased linearly as a function of luminal viscosity. There was also a pronounced decrease in the first-order emptying rate constant as lumenal viscosity increased from water to low-viscosity hydroxypropylmethylcellulose, but as viscosity was further increased there was little additional change. These results indicate that water-soluble fibers can exert a significant influence on both the lumenal viscosity and the transit profile in the upper gastrointestinal tract.