Single unitary discharges in the nodose ganglia were recorded extracellularly in chloralose anesthetized cats while amino acid solutions were being perfused through the small intestine via implanted cannulae. Test infusions consisted of either amino acid mixtures (12 amino acids; 120 mM in all) or individual amino acids (50 mM each) dissolved in Krebs Henseleit buffer. Units which were activated by amino acid infusions were also tested with 10% glucose infusions performed in the same way. Control infusions consisted of either buffer alone or a physiological saline solution isotonic to the test solution. All perfusions were performed at 38 degrees C, pH 7.4 by means of a syringe over a 10 second period. Out of 1250 vagal units activated by electrical vagal stimulation, 92 units showed an increased firing rate in response to amino acid intestinal perfusions. Of these, only 1/7 were also responsive to glucose perfusions. Osmotic, thermal or mechanical stimuli associated with infusions did not modify vagal responses to the amino acids. Among vagal units responding only to amino acid but not to glucose infusion, some were activated in a specific manner, depending on the specific amino acid infused intraduodenally. These neurons illustrated a very strict specificity regarding the nature of chemical stimuli. The very short latency, mean of 9 sec +/- 0.7 (SE) of these vagal neurons to amino acid infusions unequivocally indicates that chemoreceptors are located at the preabsorptive level. The corresponding fibers were non-myelinated (conduction velocities: 0.8/1.4 m/sec.) and were of the C type. The functional characteristics of these vagal amino acid receptors are discussed in terms of the role of intestinal signals in short term protein satiety.