Both Endomorphin 1 and Endomorphin 2 are potent analgesics with peak effects seen at 10 and 15 min, respectively. All subsequent studies are performed at peak effect. Both compounds are fully active supraspinally and spinally, with no indication of ceiling effects. Endomorphin 1 is significantly more potent spinally than supraspinally and, at the spinal level, it is significantly more potent than Endomorphin 2. The response of both agents are readily reversed by naloxone. β-FNA, a highly selective μ antagonist, effectively reverses the actions of both Endomorphins. Neither compound have analgesic activity in CXBK mice at a dose which produced over 70% analgesia in control CD-1 mice^[1].