Two antisera (Anti-P7 and Anti-P10) were raised against (-Gln-His-Pro-Gly-) elongated peptides: P7 Gln-His-Pro-Gly-Lys-Arg-Phe) and P10 (Ser-Lys-Arg-Gln-His-Pro-Gly-Lys-Arg-Phe). They recognized TRH extended peptides but not TRH. A RIA against P7 and a highly sensitive enzyme immunoassay against P10 were used to identify two major high mol wt forms of 25-35 K and 6-8 K in chromatography fractions of adult and fetal mouse as well as adult rat hypothalami. The existence of the largest form was confirmed by immunoblotting with Anti-P7. During mouse hypothalamus development in vivo and in vitro, the ratio of TRH content vs. P10-associated immunoreactivity increased several times. This suggests that these Pro-TRH peptides are precursors of TRH biosynthesis and indicate an acceleration of TRH processing during development. Double immunostaining with A-TRH and A-P7 of hypothalamic cells taken on the 16th fetal day and cultured for 6, 12, and 18 days in vitro (DIV) revealed three populations of neurons: 1) a very minor population (approximately 2%) of small round cells positive with A-TRH only; 2) a major population of neurons positive with both A-TRH and A-P7. 3) multipolar neurons positive with A-P7 only (up to approximately 45% after 18 DIV). The respective distribution of TRH and P7 along neurites also varied with time in culture. Whatever perikarya staining, TRH was restricted to short neurites and growth cones before synapse formation and, during synapse development, to varicosities and terminal boutons. However even at the latest stage examined some varicosities and terminal boutons were positive with A-P7 only. These results suggest a preferential processing of pro-TRH at a post-Golgi step during axonal transport to growth cones and synaptic boutons.