Peptides homologous to the gastro-intestinal hormones, gastrin and cholecystokinin (CCK), are present in the brain. Using immunohistochemistry we localized a neurosecretory system, reacting with antigastrin sera, in the brain of amphibians by light and electron microscopy. It has a dual location: (1) in the nucleus infundibularis ventralis (NIV) two groups of perikarya give rise to fibre tracts which join up in the infundibular floor and end in the external zone of the median eminence; (2) in the preoptic area nerve fibres and endings are found near the preoptic recess. At the electron microscope level, the immuno-colloidal-gold method revealed cells characterized by small immunoreactive dense granules, localized in the perikarya, nerve fibres and endings. Specificity controls showed that the immunoreacting substance(s) contain(s) all or part of the sequence (Trp-Met-Asp-Phe) common to gastrin, CCK and pentagastrin. The exact nature(s) and physiological function(s) of these substance(s) has (have) still to be defined.