1. Fifty-seven taste neurons were isolated in the nucleus solitary tract (NST) and tested with 15 sapid chemicals. On average, NST neurons responded well to NaCl, sucrose, monosodium L-glutamate (MSG), NaNO3, and glycine (mean = 8.2-11.0 spikes/s). Mean responses to KCl, NH4Cl, HCl, malic acid, and quinine HCl (QHCl) were low (mean = 0.7-2.9). The average responses to the other stimuli (citric acid, MgCl2, fructose, maltose, and polycose) fell between these extremes (mean = 4.3-5.1). 2. On the basis of the largest response to the four standard stimuli, the neurons were classified as follows: 15 NaCl-best, 23 sucrose-best, 17 citric acid-best, and 2 QHCl-best. 3. The NaCl-best neurons responded robustly and nearly equally to the three sodium salts (mean = 15.7-20.8) but much less so and more variably to the nonsodium, chloride salts (mean = -0.1-4.6). Sucrose-best neurons responded strongly to sucrose, glycine, and MSG (mean = 13.7-17.8), but only moderately to the other sugars (fructose and maltose) and to polycose (mean = 8.4, 9.8, and 8.8, respectively). 4. Citric acid-best neurons responded moderately to citric and malic acid (mean = 9.4 and 4.7), but less so to HCl (mean = 3.1). The two QHCl-best neurons responded moderately to QHCl and MgCl2 (mean = 12.0 and 9.5), but weakly or not at all to the other stimuli (mean = -1.1-3.1). 5. Unlike parabrachial taste neurons, none of the medullary taste cells responded specifically to Cl(-)-containing chemicals. The responses that did occur to nonsodium salts were weak and variable and often occurred in either citric acid-best or QHCl-best neurons, rather than in those that responded vigorously to sodium salts. Similar relationships have been observed in anesthetized preparations. 6. A hierarchical cluster analysis for 57 neurons across 15 stimuli produced four second-order clusters that consisted primarily of NaCl-best, sucrose-best, citric acid-best, and QHCl-best neurons, respectively. Although the analysis for neurons produced only four such clusters, a similar analysis for the 15 stimuli separated the sodium salts (NaCl and NaNO3), nonsodium salts (KCL, NH4Cl, and MGCl2, sweeteners (sucrose, maltose, fructose, and glycine), acids (citric acid and malic acid), and QHCl. 7. Monosodium glutamate activated both NaCl-best and sucrose-best neurons, but the stimulus analysis clumped it with the sodium salts.(ABSTRACT TRUNCATED AT 400 WORDS)