After intravenous injection of 125I-labeled human insulin and analogs in normal and nephrectomized rats, we examined their kinetic fate by Q-Sepharose separation into intact ligand, "fragments" (genuine fragments and protein-bound radioactivity), and iodide. Receptor binding association and dissociation constants (kass and kdis, respectively) of the analogs were estimated dynamically in vitro by BIAcore. The very fast disappearance of intact ligand from serum was found to be determined by 1) both kass and kdis of receptor-bearing tissue, thus substantiating our primary hypothesis; 2) elimination by kidneys, and 3) fast extravascularization. The rate of appearance of degradation products from receptor-mediated intracellular processing seems determined by kdis. With the possible exception of a truncated analog, ligand appears protected against degradation while the intracellular receptor-ligand complex remains intact. Non-receptor-mediated processing in kidneys is slow, compared with the receptor-mediated uptake and degradation of ligands with rate constants comparable to those of insulin. We observed binding of insulin and analogs putatively to serum proteins; binding capacity and affinity appeared insignificant for insulin but considerable for some analogs.