The importance of the dorsal vagal complex (DVC) in the control of gastric motor activity has been previously established by electrical and chemical stimulation of this region. We have further evaluated excitatory and inhibitory influences on motor activity of the gastric corpus by microinjection of L-glutamic acid (GLU) and thyrotropin-releasing hormone (TRH) into the DVC. GLU and TRH were ejected by pressure (20-30 psi) in 1-10 nl vol. from multibarreled micropipettes and intraluminal pressure in the gastric corpus was measured using a manometric catheter placed into the stomach through the pylorus of urethane-chloralose anesthetized rats. Gastric motor activity was monitored while micropipettes were advanced from the surface of the dorsomedial medulla to a depth of 1 mm in 100 micron increments. Microinjections of GLU (1-10 pmol) at depths of 200-600 microns below the surface of the brainstem caused a decrease in tonic intraluminal pressure and amplitude of phasic contractions of the gastric corpus. Injection of TRH (1-10 pmol) at depths of 200-800 microns increased both tonic intraluminal pressure and amplitude of phasic contractions. The responses to GLU (10 pmol) and TRH (10 pmol) were abolished by hexamethonium and vagotomy; atropine abolished the effect of TRH and attenuated that of GLU. It is concluded that GLU evokes only vagally mediated inhibitory effects on tonic and phasic gastric motor activity when microinjected into the DVC. In contrast, injection of TRH at the same loci causes only vagal cholinergic increases in motor activity. Subpopulations of neurons in the DVC may, therefore, be activated by specific neurotransmitters having opposite effects on gastric motor activity.