Abstract Protein biosynthesis in neurointermediate lobes of mouse pituitaries was investigated using pulse and pulse-chase techniques with [ 3H]lysine. Electrophoretic analysis of lobe homogenates on acid-urea gels resolved 11 labeled products. One was a large protein which was rapidly synthesized during pulse-incubations and disappeared during chase incubations. Three of the products increased during chase incubations, suggesting a precursor-product mode of biosynthesis for these peptides. One of these three products co-migrated with synthetic α-MSH and also corresponds to the major peak of mouse neurointermediate lobe MSH bioactivity and immunoactivity on electrophoretograms. Another case of these chase peptides has electrophoretic properties similar to those of ACTH.