Abstract N-Carbobenzoxy-S-methoxybenzyl-l-cysteinyl-l-tyrosyl-l-isoleucyl-l-glutaminyl-l-asparagin-yl-S-p-methoxybenzyl-l-cysteinyl-l-prolyl-l-leucylglycinamide (IX) was synthesized by the step-wise elongation method, and highly potent oxytocin was obtained from IX by treatment not only with sodium in liquid ammonia, but also with boiling trifluoroacetic acid, followed by the usual aeration procedure. It was demonstrated during the reactions that the newly introduced S-p-methoxybenzyl group could be cleaved more easily than the usual S-benzyl group, and that the new S-protecting group was useful in the preparation of complicated cysteinyl peptides.