1. Mechanisms of neural coding of gustatory stimuli were studied in the nucleus of the solitary tract (NTS), the first relay in the neural pathway for gustation, in anesthetized rats. Taste-responsive NTS units were identified as "relay" or "nonrelay" based on the electrophysiological response to electrical pulses delivered to the parabrachial nucleus of the pons (PbN), the second relay in the neural pathway for gustation. Coding mechanisms in each group were analyzed separately. 2. Taste responses to sapid solutions of NaCl (0.1 M), HCl (0.01 M), quinine HCl (0.01 M), sucrose (0.5 M) and Na-saccharin (0.004 M) were recorded in single units in the NTS. After gustatory stimulation, electrophysiological responses to electrical stimulation of the taste-responsive part of the ipsilateral PbN were recorded. A 0.2-ms pulse was delivered at 75-250 microA at a rates of 1, 25, 50 and 100 pps through a bipolar stainless steel electrode. An antidromic response was defined as a time-locked spike that occurred at a fixed latency after PbN stimulation that followed high stimulation frequencies. A collision test also was performed. 3. Of 42 taste-responsive NTS units, 19 (45%) were relay units, 22 (52%) were nonrelay and 1 unit was activated orthodromically by PbN stimulation. Latencies of evoked spikes ranged from 1.75 to 4.0 ms 2.1 +/- 0.2 ms (mean +/- SE, median, 1.75 ms). 4. Examination of general response characteristics revealed few differences among relay and nonrelay units.(ABSTRACT TRUNCATED AT 250 WORDS)