N-Halosuccinimides have been found to convert cysteine, S-(acetamidomethyl)-, S-(p-methoxybenzyl)-, or S-(p-methylbenzyl)cysteine into cystine. N-Iodosuccinimide, the mildest reagent among N-halosuccinimides, is applied successfully in the synthesis of (Arg 8 )-vasopressin and oxytocin by a fully automated process of solid-phase peptide synthesis or by cyclizing the released thiol peptides in DMF-CH 2 Cl 2 . These simple and synthetically useful methods have led to the fully automated solid-phase synthesis of (Cys 5 ,Cys 12 ) human growth hormone releasing factor (1-29) NH 2 and apamin based on the combined processes of stepwise addition of Boc-amino acids and controlled stepwise formation of the disulfide bonds