The regulation of thyrotropinreleasing hormone (TRH) secretion by putative neurotransmitters was studied in vitro by measuring the release of radioactive TRH from mouse hypothalamic fragments which had been pulselabelled with tritiated TRH precursor amino acids. Both dopamine and norepinephrine, when added to the incubation media in a concentration of 10-4M or 1O-5M, increased the release of 3H TRH. The TRH stimulatory effects of dopamine, but not of norepinephrine were abolished by Disulfiram, a β-oxidase inhibitor, thus suggesting that dopamine was effective after conversion to norepinephrine. Release of 3H-TRH was decreased upon addition of serotonin at 10-4M. Acetylcholine in the presence of either Eserine or the acetylcholine analogue, carbachol, had no effect on TRH release. These findings suggest that the regulation of TRH release is under a dual monoaminergic control system with a positive noradrenergic and a negative serotoninergic component. (Endocrinology93: 626, 1973)