A multichannel tube with two occluding balloons was developed for measurements of the secretion of different endogenous substances into a defined segment of the small intestine. The tube was inserted under fluoroscopic guidance with the aid of a guide wire and was in position in the proximal jejunum after 45-90 min. The overall success rate of the complete procedure was 90%. The system prevented contamination of the perfusate from gastric and pancreatic secretion. Major dilution of the intestinal secretion was avoided due to high recovery of the perfusion fluid. The jejunal secretion rates of high and low molecular weight substances, i.e., albumin, beta 2-microglobulin, hyaluronan, and histamine, were tested. The recovery of the effluent fluid and the volume marker 14C-PEG 4000 exceeded 95% and was also stable over the perfusion period. The mean secretion rates in 23 healthy controls of albumin (635 micrograms/cm/h), beta 2-microglobulin (0.87 microgram/cm/h), hyaluronan (1061 ng/cm/h), and histamine (70 ng/cm/h) were also stable during the perfusion period. Ten healthy controls were studied under steady state conditions for 3 h. The concentrations in jejunal perfusion fluid relative to serum levels were for albumin 0.06%, indicating the degree of passive leakage from the plasma compartment to the intestinal lumen. The appearance of the low-molecular weight protein beta 2-microglobulin in perfusion fluid was on average 3% of its circulating levels, suggesting that the concentrative transport from plasma is dependent on the molecular size. The jejunal fluid concentrations of histamine and hyaluronan exceeded their respective concentrations in plasma, indicating that local intestinal secretion/synthesis is mainly responsible for their appearance in jejunal fluid. In summary, the technique is simple, rapid, atraumatic, safe, and reproducible and, potentially, can accurately reflect biochemical processes in the small intestine.