SUMMARY1. Synaptic transmission and neuronal morphology were studied in the nucleus tractus solitarius and in the dorsal vagal motor nucleus (solitary complex), in coronal brain-stem slices of rat or cat, superfused in vitro.2. Electrical stimulation of afferent fibres of the solitary tract evoked two different types of post-synaptic response recorded intracellularly in different solitary complex neurones. Labelling with horseradish peroxidase showed that these two sorts of orthodromically evoked responses were correlated with different post-synaptic neuronal morphologies.3. The majority of recorded neurones (n = 93) showed a prolonged reduction in excitability following the initial solitary-tract-evoked excitatory post-synaptic potential (e.p.s.p.). A smaller number of neurones (n = 53) showed a prolonged increase in excitability following solitary tract stimulation. In no case did the solitary tract stimulation induce a burst of action potentials at high frequency.4. The time-to-peak and the half-width of the initial solitary-tract-evoked e.p.s.p. were shorter in neurones with prolonged increased excitability than in those with prolonged reduced excitability. In neurones with prolonged reduced excitability, this e.p.s.p. was followed by a hyperpolarization lasting 60-100 ms. The latency of this inhibitory post-synaptic potential (i.p.s.p.) was 3-5 ms longer than that of the initial e.p.s.p. and its reversal potential was 10 mV more negative than the reversal potential of the response measured following application of y-aminobutyric acid or glycine. In neurones with prolonged increased excitability, at a membrane potential of -40 to -50 mV, the initial solitary tract e.p.s.p. was followed by a prolonged depolarization lasting 100-400 ms.5. Background synaptic activity was high in neurones with prolonged increased excitability, consisting of unitary e.p.s.p.s with an amplitude of more than 0-8 mV. This activity was increased for a period of 300-800 ms following solitary tract stimulation. Spontaneous excitatory potentials of more than 0-5 mV were not seen in neurones with prolonged reduced excitability. In these neurones, after intracellular injection of choride ions, reversed unitary i