The therapeutic potential of opioid receptor agonists is often limited by their high propensity to cause tolerance after chronic administration. One of the possible mechanisms responsible for the development of tolerance is the agonist-dependent modulation of receptor internalization. Based on our discovery of a D-amino acid rich tetrapeptide from Penicillium bilaii, we have developed a structurally novel μ-opioid receptor selective (binding Ki = 1.5 nM (μ), 300 nM (δ) and 1000 nM (κ)) agonist (Bilaid C2) that fails to produce receptor internalization in contrast to other peptidic agonists endomorphins 1 & 2, Met-enkephalin and DAMGO. Bilaid C2 produced analgesia comparable to that of morphine when administered intrathecally (3 μg) however it failed to produce antinociception subcutaneously (s.c.) at doses as high as 100 mg/kg, indicating poor blood-brain-barrier (BBB) penetration. With the aim of improving BBB permeability, we have made further chemical modifications to Bilaid C2 including the introduction of a saccharide, a helical penetrating sequence, as well as enzymatically and/or chemically reversible moieties to mask the N-terminus. A number of these analogues produced analgesic effects through s.c. administration at doses comparable to that of morphine. These compounds are potential candidates for the development of s.c. injectable analgesics that may have altered tolerance and dependence inducing properties.