The effect of various putative neuro-transmitters upon the release of corticotrophin releasing hormone (CRH) was investigated using the hypothalamus of the rat in vitro. Acetylcholine (1–5 pg) caused a dose-dependent release of CRH which was antagonized by hexamethonium (1–10 ng) and partially antagonized by atropine (300 pg). Neither noradrenaline, dopamine nor histamine had any effect on the basal secretion of CRH. Noradrenaline (10 ng), however, was able to inhibit the release of CRH in response to acetylcholine (3 pg) and this action of noradrenaline was reduced by phentolamine (100 ng), an α adrenergic blocking a