The alpha-melanocyte stimulating hormone (α-MSH) variants from rat and human pituitary and rat brain have been analyzed in detail by radioimmunoassay (RIA) interfacing with various chromatography systems. No authentic α-MSH was detected in extracts of rat brain regions or human pituitary. In these tissues the only α-MSH variant detected was characterized as des-acetyl α-MSH. Conversely, acetylated α-MSH was found to be the predominant form in rat pituitary. In addition to α-MSH, extracts from both rat and cow pituitary were found to contain an α-MSH variant with the characteristics of an O-N-diacetyl derivative. © 1982.