Concentrations of methionine-enkephalin- (Met-enkephalin) and beta-endorphin-like immunoreactivities were determined in 33 areas of human brain and pituitary using highly sensitive radioimmunoassays in combination with affinity chromatography for the purification of beta-endorphin. It was found that they have quite different distribution patterns, suggesting the existence of both endorphins in independent systems in the central nervous system. Determination of Met-enkephalin and beta-endorphin immunoreactivities in chronic alcoholics and opiate-dependent subjects revealed no gross changes in comparison to the normal subjects.