The hypothalamus weighs only a few grams at the base of the brain, yet it sends releasing hormones to the pituitary, thereby controlling growth hormone, thyroid‐stimulating hormone and many others. Guillemin and Schally collected hundreds of kilograms of sheep and pig brains, performed repeated chemical fractionations and chromatography, and purified tiny amounts of active substances. They showed that thyrotropin-releasing hormone (TRH) consists of just three amino acids and that gonadotropin-releasing hormone (GnRH) contains ten, both exerting powerful control over pituitary secretion. Their work established the concept of neuroendocrinology—that the brain uses chemical messengers, not only electrical signals, to govern the whole body—bridging endocrinology and neuroscience. The knowledge directly led to drugs used for ovulation induction in IVF procedures and for prostate-cancer therapy. In society it is applied to infertility treatment, breast-cancer screening, anti-doping tests and many other fields.

Guillemin and Schally combined acetone–sulfate precipitation, ion-exchange, gel filtration and reversed-phase chromatography while tracing bioactivity with rat pituitary assays to isolate various releasing hormones. Demonstration that synthetic TRH (pGlu-His-Pro-NH2) possessed identical activity proved conclusively that hypothalamic hormones are peptides. They also established that GnRH is the decapeptide pyroGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 and is highly conserved across mammals. These findings supplied the molecular basis for the three-tier hypothalamic–pituitary–peripheral endocrine feedback axis. Subsequent cloning research identified the GnRH receptor and associated G-proteins, clarifying second-messenger signal transduction. Today synthetic GnRH agonists and antagonists are indispensable in treating breast cancer, endometriosis and precocious puberty, making this work a textbook example of basic research leading directly to clinical application.

Both groups achieved 10^6–10^9-fold concentration by sequential chromatography and identified activity peaks with chicken pituitary suppression assays and in vitro 125I-labelled ligand binding. TRH was elucidated by automated peptide analysis and Edman degradation, and C-peptide-like sequences suggested the presence of prohormone precursors. For GnRH, extensive FMOC solid-phase synthesis and mass spectrometry yielded analogues that revealed Tyr5, Gly6 and Leu7 as critical for receptor affinity and intrinsic activity, enabling rational design of super-agonists (leuprorelin) and antagonists (cetrorelix). Clinical studies employing pulsatile GnRH infusion demonstrated that pulse frequency and amplitude are the key determinants of gonadal function, corroborating a hypothalamic pulse generator. The work laid the methodological groundwork for neuropeptide research, profoundly influencing later identifications such as neuropeptide Y and orexin.