40 pmol of biosynthetic human proinsulin was administered to 8 healthy volunteers by intravenous and by subcutaneous route. Following proinsulin administration, venous blood was collected in regular intervals within which proinsulin was determined by a specific radioimmunometric assay with monoclonal antibodies. The proinsulin concentration was determined simultaneously with the insulin and C-peptide radioimmunoassay. Through this investigation the following kinetic parameters were found: The kinetics of the biosynthetic human proinsulin can be best described by the 3-compartment model. The dominant biological half-life was 92 minutes. In intravenous proinsulin administration a proinsulin mean transit time of 80 minutes was found, whereas in subcutaneous administration a proinsulin retention time of 225 minutes was measured. The mean resorption velocity of the subcutaneously applied proinsulin amounted to 145 minutes. Two lag times for subcutaneous resorption can be described, a short one with 9.4 minutes and a long one with 65 minutes. The initial distribution volume for proinsulin was 3.8 l, whereas the distribution volume after complete distribution was 9.3 l. The mean total metabolic clearance was determined with 120 ml/min. Since no difference for the proinsulin concentration was found using the 3 different determination methods a peripheral proinsulin conversion to insulin and C-peptide is not likely. The basal endogenous secretion rate for proinsulin is 68.7 pmol per hour.