Recent evidence indicates that the cell bodies of vagal cardioinhibitory neurons are located principally in the external formation of the nucleus ambiguus (NA). As activation of baroreceptor afferent fibers projecting to the nucleus tractus solitarius (NTS) elicits a decrease in heart rate it is likely that there is a connection between the NTS and NA. To test the hypothesis that stimulation of the NTS can excite vagal preganglionic cardiomotor neurons (VPCN) in the NA, activity from 78 neurons in the NA was recorded extracellularly before and during stimulation of a depressor site in the NTS (1 Hz, 0.1 ms) in urethan anesthetized and artificially ventilated male Wistar rats. Sixteen neurons were characterized as vagal preganglionic cardiomotor neurons (VPCN) because they were excited by baroreceptor activation (1-3 micrograms phenylephrine i.v.) and showed rhythmicity of their spontaneous activity in synchrony with the cardiac cycle. Stimulation of the NTS increased the firing rate of all these VPCN. The remaining 62 neurons could not be considered as VPCN because they either had respiratory rhythmicity or were not sensitive to baroreceptor activation, or they were sensitive to baroreceptor activation but did not display cardiac cycle related rhythmicity. These results provide evidence for the existence of an excitatory pathway from NTS to vagal preganglionic cardiomotor neurons in the NA.