Glycinergic mechanisms have been implicated in central cardiovascular regulation. However, the inhibitory amino acid's role in the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferents, has not been clarified. Thus, we sought to determine if the microinjection of glycine into the NTS alters arterial pressure and heart rate. Microinjections of glycine, like glutamate, confined to the NTS decreased arterial pressure and heart rate in a neurally mediated, dose-dependent manner. The glycine antagonist strychnine completely blocked these effects of glycine but did not itself alter arterial pressure or heart rate, or interfere with the baroreceptor reflex. The acute hypotensive, bradycardic response to glycine was followed by a period during which glycine essentially eliminated the cardiovascular responses to the microinjection into the NTS of glutamate, an amino acid reputed to be a transmitter in the baroreceptor reflex arc. These data suggest that glycine is involved in cardiovascular regulation by the NTS but do not support its being an integral transmitter in the baroreceptor reflex.