Vagal afferents ascending from the gastrointestinal tract synapse on neurons in the nucleus of the solitary tract. Although these neurons constitute a significant proportion of solitary tract cells their firing behaviour and synaptic properties are not documented. Since gastrointestinal tract afferent termination sites overlap with regions mediating cardiorespiratory reflexes the possibility of convergence with afferents mediating cardiovascular and respiratory reflexes was proposed. Here we describe some electrophysiological and morphological properties of solitary tract neurons orthodromically driven from the subdiaphragmatic vagus nerves and assess possible convergent inputs from cardiorespiratory afferents. Whole-cell recordings of solitary tract neurons responding to electrical stimulation of the sub-diaphragmatic vagus nerves (0.1-1 ms; 1-10 V; 2-20 Hz) were made in a working heart-brainstem preparation of rat. Baroreceptors were stimulated by raising pressure in the aorta or carotid sinus, whereas aortic injection of sodium cyanide (0.05% solution 25-50 microl) was used to activate peripheral chemoreceptors. Phrenic nerve activity and heart rate were monitored continuously. Of 88 solitary tract neurons tested, 39 responded with an evoked excitatory synaptic potential following stimulation of the sub-diaphragmatic vagus nerves. Resting membrane potential and input resistance of sub-diaphragmatic vagus nerve driven solitary tract neurons were 53.2 +/- 0.5 mV and 291 +/- 17 Mohms, respectively (mean +/- S.E.M.). Response latencies to sub-diaphragmatic vagus nerve stimulation were divided into two groups: <20 ms (16.0 +/- 2 ms, n = 7; mean +/- S.E.M.) and >20 ms (77.3 +/- 5 ms, n = 32). One additional neuron displayed an evoked inhibitory postsynaptic potential (latency 175 ms). Nineteen neurons showed ongoing activity which consisted of either irregular single action potential firing (0.5-10 Hz; n = 12) or burst discharge (n = 7). Of 33 neurons tested, 17 showed spike frequency adaptation during injection of positive current, whereas 19 of 38 cells displayed rebound excitation following release from hyperpolarized potentials. There was no correlation between these properties and synaptic latencies. Ninety-one per cent of neurons tested displayed synaptic depression following paired pulse stimulation of the sub-diaphragmatic vagus nerve over intervals up to 500 ms. Stimulation of either baroreceptors (n = 31) or chemoreceptors (n = 36) failed to elicit a synaptic response in all sub-diaphragmatic vagus nerve-driven solitary tract neurons. Neurobiotin-labelled solitary tract neurons (n = 10) were from both latency groups and were located medial to the solitary tract at the level of area postrema, -0.3 mm to +1 mm from the obex. One cell was located in commissural subnucleus at midline, seven cells dorsal to the tractus solitarius and three ventral and medial to it. Soma sizes were 23 +/- 9.6 x 14 +/- 4.9 microm (range: 50 x 16 microm to 15 x 7 microm). The number of primary dendrites varied from three to five, secondary from one to eight and tertiary zero to four. Labelled axons were found in seven cells which ramified extensively in the solitary tract nucleus (n = 3) and/or branched extensively in the dorsal vagal motonucleus (n = 3) and/or projected towards the ventrolateral medulla (n = 3). We conclude that solitary tract neurons receiving signals from the sub-diaphragmatic vagus nerves (most likely from gastrointestinal tract structures) appear to be a distinct pool of neurons. There was a heterogeneity in terms of both their ongoing activity and projection targets but despite this, there were three consistent properties. First, sub-diaphragmatic vagus nerve evoked predominantly excitatory synaptic responses in solitary tract neurons; second, neurons exhibited lasting paired pulse depression following activation of sub-diaphragmatic vagus nerves; and third, sub-diaphragmatic vagus nerve-driven solitary tract neurons were