Release of endogenous glutamate, γ-aminobutyric acid (GABA) and glycine was investigated in the caudal dorsomedial (CDM) region of the rat medulla oblongata. High potassium stimulation (50 mM) produced an increase in release of glutamate, GABA and glycine by 579.1, 303.4 and 774.5 (pmol/mg protein), respectively. The release was abolished by Ca²⁺ deprivation. These results indicate a possible neurotransmitter role for glutamate, GABA and glycine in the CDM medulla of the rat.