Abstract Five NH 2 -terminal extension analogs of methionine-enkephalin have been synthesized by solid phase methodology and their morphinomimetic activity determined in the guinea pig ileum-myenteric plexus preparation. Relative to methionine-enkephalin which corresponds to β-LPH-(61–65) and arbitrarily assigned a potency value of 100, β-LPH-(60–65), β-LPH-(59–65), β-LPH-(58–65), β-LPH-(57–65) and β-LPH-(56–65) have potencies of 90, 7, 4.4, 0.3 and 0.11 respectively.