U-50,488H is a chemically novel analgesic that is a potent opioid-like agent on the mouse tail flick and electrically stimulated guinea pig ileum tests. U-50,488H is a very weak competitor for naloxone binding sites in brain and ileum. However, the drug has high affinity for kappa receptor binding sites revealed by competition for EKC sites in the presence of dihydromorphine. Morphine has both supraspinal and spinal sites of action since it was a potent analgesic after both intracranial and intraspinal injections. However, U-50,488H works predominantly at the spinal level. Dynorphin may be an endogenous ligand at this site. Studies on cat dorsal horn neurons suggest that U-50,488H analgesia may be due to an increase in threshold for neuron excitation.