Pentapeptides with analgesic activity on peripheral administration

A pentapeptide of the formula:

• R.sup.1 --N--H--Tyr--N--R.sup.2 --D--Met--Gly--N--R.sup.3 --Phe--N--R.sup.4 --D--Ser--X
• wherein
• R.sup.1 is hydrogen, methyl, ethyl, propyl, 2-methyl-2-pentenyl, 2-methyl-1-pentenyl, methylcyclopropyl, or methylcyclobutyl;
• R.sup.2, R.sup.3, and R.sup.4 are, independently, hydrogen or methyl;
• and
• X is an amino group, -OR.sup.5, or -CH.sub.2 OR.sup.5 where R.sup.5 is hydrogen or an alkyl group containing 1 to 6 carbon atoms;
• or a pharmaceutically acceptable salt thereof.