A third melatropin fragment named gamma-MSH has been described in the N-terminal portion of the common precursor of bovine ACTH and beta-LPH by Nakanishi et al. (Nakanishi, S., Inoue, A., Kita, K., Nakamura, M., Chang, A.C.Y., Cohen, S.N. and Numa, S., Nature, 278 (1979) 423-427). In order to determine if immunoreactive gamma-MSH was present in the rat pituitary gland and to accurately localize this peptide, an immunocytochemical localization of gamma-MSH was conducted at both light and electron microscopic levels. Specific immunostaining was detected in stellate cells scattered throughout the pars distalis and in all the cells of the pars intermedia. At the ultrastructural level, immunoreactive gamma-MSH was only observed in the lipocorticotrophs. Using serial ultrathin sections, it was shown that the secretory granules which contain ACTH were also labeled for gamma-MSH. These results suggest that fragment(s) of the common precursor of ACTH and beta-LPH and/or the whole common precursor is released with peptides of known biological activity.