Sympathetic preganglionic neurons (SPNs), located in the intermediolateral cell column (IML) of the thoracolumbar spinal cord, contribute to the maintenance of arterial pressure (AP) and heart rate (HR) within normal limits under different physiological conditions. The purpose of this study was to examine the effects of microinjecting the putative inhibitory transmitters glycine (GLY) or gamma-amino-butyric acid (GABA) into functionally identified cardioacceleratory and vasoconstrictor IML sites at T1-T3. Rats were anesthetized (1.4 g/kg urethane i.p.), paralysed with decamethonium bromide (3.3 mg/kg i.v.) and artificially ventilated. Glutamate (GLU) microinjection (10-20 nl, pH = 7.4, 0.15 M in phosphate-buffered saline (PBS)) was used to identify 29 vasoconstrictor sites, of which 23 were also cardioacceleratory, in the right side of the T2 segment. GLY microinjection (10-20 nl, pH = 7.4, 0.5 M in PBS) into these sites resulted in significant decreases in both AP (n = 18) and HR (n = 14). The AP and HR responses to GLY were brief in duration and were attenuated by the specific GLY antagonist strychnine (STR). Microinjection of GABA (10-20 nl, pH = 7.4, 0.15-0.84 M in PBS, n = 6) and its agonist muscimol (10-20 nl, pH = 7.4, 0.9 mM in PBS, n = 6) into GLU-identified sites in the IML caused no changes in AP or HR. However, after the application of either GABA or muscimol, the changes in AP or HR elicited by GLU were eliminated, suggesting that GABA and muscimol decrease the excitability of SPNs.(ABSTRACT TRUNCATED AT 250 WORDS)