A procedure to collect pure pancreatic juice endoscopically is described which allows an instant estimation of the protein content of the secretion by measuring its absorption at 280 nm. By this the kinetics of protein secretion can already be read from a curve during the collecting procedure, thus avoiding a multitude of small fractions otherwise necessary. Timing and size of the samples are adjusted individually according to the course of protein secretion and not to a rigid regimen. The method described is especially apt to investigations of pancreatic proteins and substances the secretion of which parallels that of protein; e. g. trypsinogen, calcium and zinc, however not sodium and potassium.