In this paper we describe the synthesis and affinity toward the mu-opioid receptor of some tetrapeptides obtained from endomorphin-1, H-Tyr-Pro-Trp-Phe-NH(2) (1), by substituting each amino acid in turn with its homologue. The ability to bind mu-opioid receptors depends on the beta-amino acid, and in particular 4, which contains beta-L-Pro, has a K(I) in the nanomolar range. The peptides 4 and 5 are significantly more resistant to enzymatic hydrolysis than 1. The same compounds, as well as the mu-opioid receptor agonist DAMGO, produced a concentration-dependent inhibition of forskolin-stimulated cyclic AMP formation, thus behaving as mu-opioid agonists. These features suggest that this novel class of endomorphin-1 analogues may represent suitable candidates for the in vivo investigation as potential mu-opioid receptor agonists.