The solid-phase synthesis of human β-endorphin is described. A yield of 32% is achieved based on starting resin. The synthetic product behaves as a homogeneous peptide in partition chromatography, paper electrophoresis, thin-layer chromatography, disc electrophoresis, amino acid composition, and a tryptic map. The synthetic βh-endorphin possesses antinociceptive properties as estimated by the tail-flick, hot-plate, and writhing tests in mice. When applied centrally, βh-endorphin is 17-48 times more potent than morphine. It is 3.4 times more potent than morphine when injected intravenously. The analgesic responses are blocked by the specific opiate antagonist, naloxone.