Microinjections of gamma-aminobutyric acid (GABA) and glycine, into the medial area of the nucleus tractus solitarii (NTS) of the rat, led to an increase in arterial pressure and heart rate. The GABA receptor antagonist bicuculline and the glycine receptor antagonist strychnine decreased both of these cardiovascular parameters whereas the GABA uptake inhibitor nipecotic acid produced hypertension. High K+ stimulation caused a calcium-dependent release of GABA and glycine from tissues in the area of the NTS. Our results suggest that GABA and glycine may modulate the cardiovascular control within the NTS.